Time trends in patients managed simultaneously in multiple hospital outpatient specialty clinics for chronic diseases: A register-based cross-sectional study

Abstract

Background:

The increasing prevalence of patients with multimorbidity in the general population affects the health-care system. There is a lack of knowledge of the proportion of patients attending multiple hospital outpatient specialty clinics simultaneously.

Objective:

This study describes the development in the proportion of patients managed simultaneously in multiple hospital outpatient specialty clinics.

Design:

We obtained three cross-sectional samples from all of the hospitals in Denmark. The data set consists of adults (age 18+) in long-term outpatient care on January 1 in 2004, 2009, and 2014 with one or more of 50 consensus-selected chronic diseases. Descriptive statistics were used to examine and compare the proportion of patients treated simultaneously in multiple outpatient specialty clinics. We also investigated the most common combinations of outpatient specialty clinics.

Results:

In 2004, 176,786 patients with chronic diseases were registered as receiving outpatient care in Denmark. This figure increased to 246,542 patients in 2009 and 341,015 in 2014. The proportion of patients managed simultaneously in multiple outpatient specialty clinics was 4.0% in 2004, 5.5% in 2009, and 7.7% in 2014. The most common specialty clinic combination was endocrinology and cardiology, accounting for 12.1% in 2004, 11.5% in 2009, and 9.6% in 2014.

Conclusions:

The proportion of patients in multiple clinics nearly doubled over a 10-year period. While there were some common specialty clinic combinations in which patients were treated most often, there was also considerable variation. Further studies are needed to identify generic and disease-specific initiatives.

Keywords

Multimorbidity outpatient clinics hospital

Background

As the prevalence of patients with multimorbidity (MM) increases in the population, the secondary health sector can expect an increase in the management of these patients. However, knowledge about how this MM patient population is managed in the hospital outpatient clinics is sparse.

MM is commonly defined as the presence of two or more chronic medical diseases in an individual.^1,2 A Danish population study has reported that around 50% of the adult population has one chronic disease and approximately half of these patients live with MM.³ MM increases with age, as shown in a Scottish national primary care survey in which 11% of people aged 25–44 years, 65% of those over age 65, and 82% over age 85 suffered from MM.⁴ The prevalence of MM increases with time, as shown in studies from the Netherlands, where it rose by 3.2% over a 10-year period (from 14.3% in 2001 to 17.5% in 2011)⁵ and is expected to increase further in the future as a result of aging populations together with health-care advances.^5
–7

The increasing prevalence of MM patients in the population affects all health-care sectors. While studies on the prevalence and patterns among MM patients have been conducted in the primary health-care sector in different countries,¹ no studies have been performed at hospital-level outpatient clinics.

In contrast to the primary health-care level, the hospital sector is built on single-disease management,⁸ and care for each disease is provided from disease-specialized outpatient clinics. MM patients must therefore often go to multiple clinics. To optimize clinical organization and patient trajectories, it is important to know the extent of this development among patients simultaneously managed in different outpatient specialty clinics and which combinations of specialty clinics are most common.

On that background, this study describes the population of patients managed simultaneously in multiple outpatient specialty clinics among patients in long-term outpatient treatment for chronic diseases. More specifically, we address the following two research questions: Firstly, what was the development in the proportion of patients managed simultaneously in multiple outpatient specialty clinics among outpatients between 2004, 2009, and 2014? And secondly, what were the most common specialty clinic combinations?

Material and methods

This study was designed as a historic, register-based, cross-sectional study of a national data set from Danish hospitals, extracting data for 2004, 2009, and 2014.

The Danish health-care system

The Danish Health Authority provides access to tax-financed health care for all 5.8 million persons residing in Denmark.⁹ General practitioners (GPs) provide most of the care for patients with chronic diseases.^10,11 If the chronic disease reaches a certain level of severity or complexity, however, patients are referred to specialists in hospital outpatient specialty clinics. In the hospital outpatient clinics, patients are enrolled in a course of treatment within a specific disease-group clinic for each disease for which they are referred to hospital treatment. A course of treatment consists of all the contacts in connection with diagnosing or diagnosing and treatment aspects in a hospital outpatient clinic. The duration of a treatment course is from the patient’s first contact to the clinic until the last contact, which is when treatment courses are completed or of other reasons terminated.

Data sources

All Danish hospitals, both public and privately owned, automatically report all inpatient and outpatient contacts to The National Patient Register (NPR). The NPR possesses longitudinal data derived from patient medical records on inpatient contacts in Danish hospitals since 1977 and outpatient contacts since 1995, as well as discharge diagnosis codes according to the International Classification of Diseases, Revision 8 (ICD8) for 1977–1993 and the International Classification of Diseases, Revision 10 (ICD10) from 1994 onward. Since 2000, it has also served as the basis for payment via the Diagnostic Related Group system.^12,13 The NPR was used to extract the study population and to obtain information on the patients’ age, sex, and diagnoses.

Study population

We included patients aged 18 and older who were in an ongoing course of treatment in hospital outpatient specialty clinics for one or multiple chronic diseases on January 1, 2004; January 1, 2009; or January 1, 2014. There is no standard approach for defining which diseases should be included in MM patient studies.^2,14,15 The included morbidities in such studies are therefore inevitably partly subjective and dependent on the available data. As others, we included diseases with a high prevalence in our setting.¹⁴

Information on diseases based on activity (outpatient visits) in hospital outpatient settings is available from The Danish National Board of Health statistics.¹⁶ We extracted information from 2014 from all Danish hospitals (not including psychiatric hospitals), all age groups, and based on four-character ICD-codes (e.g. DJ44).

Four specialists (two in internal medicine, one in family medicine, and one in public health) reviewed all of the diagnoses and extracted 50 chronic, frequent, and clinically relevant diagnoses. Disagreement was resolved by discussion and consensus. The 50 most frequent diagnoses constituted 31% of the outpatient group and 33% of the outpatient activity based on 2014 data.

We included patients with 1 of the 50 selected chronic diseases if they had been enrolled in a course of hospital outpatient treatment for at least 6 months from the inclusion date in 2004, 2009, or 2014. The 6-month criterion was chosen to avoid acute or subacute forms of certain diseases and transitory hospital-outpatient treated diseases. Six months duration is the definition for chronic disease used by the Danish Health Authority.¹⁷ We then categorized these 50 diagnoses into 16 organizational groups according to how Danish hospitals generally group the different specialist clinics based on information from four typical Danish hospital organizations and Danish Health Statistics.¹⁸ For instance, diabetes and thyrotoxicosis are both treated in an outpatient specialty clinic for endocrine diseases. Diagnoses and categorization are shown in Appendix 1.

Statistics

Patients in 2004, 2009, and 2014 were described in absolute numbers and percentages using 2004 as reference. The χ ² test was used to investigate significant differences in sex, age groups, and number of specialty clinics between 2014, 2009, and 2004.

To determine the most prevalent combination patterns of hospital outpatient specialy clinics, we calculated all of the possible combinations of specialty clinics and their frequencies when patients were in multiple clinics, presented as absolute numbers and percentages. The five most common specialty clinic combinations were ranged for patients treated in two clinics. Two-sided statistical tests were used, and p < 0.05 was considered statistically significant. All data analyses were performed using Stata software, version 15.0 (StataCorp LP, College Station, Texas, USA).

Ethics

The study did not involve any direct contact with patients or access to patient files, for which reason neither informed consent nor ethical approval were required.¹⁹ The study was registered with the Danish Data Protection Agency (J.nr. 18/35695). We applied STROBE criteria to ensure adequate reporting and transparency.²⁰

Results

In 2004, 176,786 patients (3.3% of the Danish population) with one or more of the 50 selected diseases were managed in a hospital outpatient setting, 246,542 patients (4.5%) in 2009, and 341,015 patients (6.1%) in 2014 (Table 1). There was an increase for almost all diagnoses. In 2014, four diagnoses (type 2 diabetes, breast cancer, type 1 diabetes, and sleeping disorders) covered 25% of the diagnoses. The changes from 2004 to 2009 and 2014 within each diagnosis group are presented in Appendix 2.

Table 1.

Study population and Danish population in 2004, 2009, and 2014.

	Study population^a		Danish population^a		Study population as proportion of the Danish population
	n	%	n	%	%
2004	176,786	100	5,398,000	100	3.3
2009	246,542	139	5,511,000	102	4.5
2014	341,015	193	5,627,000	104	6.1

^a2004 was used as the reference from which the percentage in 2009 and 2014 was calculated.

The patient characteristics for all three populations are described in Table 2. In 2004, the median age was 57 years (interquartile range (IQR) 44–69) and 34% of the patients were 65 years or older; in 2009, the median was 60 years (IQR 47–70), 38% of patients were 65+; in 2014, the median was 62 years (IQR 48–72; p < 0.001, reference 2004), 44% of patients were 65+. The proportion of patients treated in multiple outpatient specialty clinics was 4.0% in 2004, 5.5% in 2009, and 7.7% in 2014. The patients treated in one hospital specialty clinic nearly doubled over 10 years (169,678 patients in 2004 vs. 314,762 patients in 2014), while the number of patients treated in two or more specialty clinics increased nearly fourfold (7108 patients in 2004 vs. 26,253 patients in 2014).

Table 2.

Number of patients distributed by sex, age groups, and number of outpatient specialty clinics in 2004, 2009, and 2014.

	2004		2009		2014
	Patients		Patients		Patients
	n	%	n	%	n	%	p Value^a
Total number of patients	176,786	100.0	246,542	100.0	341,015	100.0
Sex							<0.02
Men	81,959	46.4	117,143	47.5	163,091	47.8
Women	94,827	53.6	129,399	52.5	177,924	52.2
Age group (years)							<0.001
18–29	13,354	7.5	14,733	6.0	19,809	5.8
30–39	19,346	10.9	22,649	9.2	27,542	8.1
40–49	25,985	14.7	34,371	13.9	44,661	13.1
50–59	39,210	22.2	47,719	19.4	61,397	18.0
60–69	36,866	20.9	60,662	24.6	85,078	24.9
70–79	29,622	16.8	45,153	18.3	68,086	20.0
80+	12,403	7.0	21,255	8.6	34,442	10.1
Number outpatient specialty clinics attended							<0.001
1 clinic	169,678	96.0	232,941	94.5	314,762	92.3
2 clinics	6829	3.9	12,896	5.2	24,312	7.1
≥3 clinics	279	0.1	705	0.3	1941	0.6

^a χ ² test for group differences between 2004 and 2009 as well as between 2009 and 2014.

The distribution of specialty clinics among patients managed in multiple clinics is shown in Table 3. The patients were most often treated at an endocrine clinic in combination with another clinic. Among patients treated in two specialty clinics, the five most common combinations of specialties are shown in Table 4. The top five pairs accounted for 31% of all combinations in 2004, 29% in 2009, and 28% in 2014. The most common combination was endocrinology and cardiology, accounting for 12.1% in 2004, 11.5% in 2009, and 9.6% in 2014.

Table 3.

Frequency distribution of specialty clinics among patients in multiple outpatient treatment in 2004, 2009, and 2014.

	2004		2009		2014
	Patients		Patients		Patients
Specialty clinic	n	%^a	n	%^a	n	%^a
Total	7108	—	13,601	—	26,253	—
Endocrinology	3407	48	6365	47	11,432	44
Cardiology	2278	32	4668	34	9005	34
Ear, nose, and throat	1421	20	2689	20	5666	22
Rheumatology	1151	16	2092	15	3975	15
Oncology	868	12	1477	11	3503	13
Pulmonary	567	8	1405	10	3114	12
Nephrology	424	6	1105	8	3027	12
Gastroenterology	801	11	1424	10	2819	11
Ophthalmology	686	10	1395	10	2789	11
Osteoarthrosis	1001	14	1539	11	2320	9
Neurology	674	9	1168	9	2011	8
Urology	329	5	979	7	1816	7
Dermatology	313	4	628	5	1292	5
Vascular surgery	207	3	432	3	683	3
Hematology	158	2	281	2	571	2
Back	218	3	301	2	537	2

^aThe total percentage is not equal to 100% as patients with several treatment courses occur under several specialty clinics.

Table 4.

Top 5 most common pairs of specialty clinics among patients with two hospital outpatient treatment courses in 2004, 2009, and 2014.

	2004				2009				2014
		Patients				Patients				Patients
		n	%			n	%			n	%
Top 5		6829				12,896				24,312
1.	Endocrinology and cardiology	825	12.1	1.	Endocrinology and cardiology	1484	11.5	1.	Endocrinology and cardiology	2333	9.6
2.	Endocrinology and ENT	382	5.6	2.	Endocrinology and ENT	673	5.2	2.	Endocrinology and ENT	1351	5.6
3.	Endocrinology and ophthalmology	320	4.7	3.	Cardiology and ENT	579	4.5	3.	Cardiology and ENT	1147	4.7
4.	Endocrinology and rheumatology	301	4.4	4.	Endocrinology and rheumatology	549	4.3	4.	Endocrinology and nephrology	988	4.1
5.	Cardiology and ENT	258	3.8	5.	Endocrinology and ophthalmology	431	3.3	5.	Endocrinology and rheumatology	889	3.7
Total			30.5				28.8				27.6

ENT: ear, nose, and throat.

Discussion

This study has shown that patients managed simultaneously in multiple hospital outpatient specialty clinics accounted for 7.7% in 2014, nearly double as many as in 2004. The most common specialty clinic combination was endocrinology and cardiology, but there was great diversity in the combinations of specialty clinics, and the top 5 specialty pairs accounted only for about one-fourth of all combinations.

Our main and novel finding is that the proportion of patients in multiple clinics nearly doubled over the last 10 years. In the existing literature, a time trend in this setting of hospital outpatient specialty clinics has not been studied before. This is probably because research in hospitals has traditionally had a single-disease or narrow-comorbidity focus, and MM in hospital outpatient settings is a relatively new research area. We found two studies from the primary health-care sector examining a time trend.^5,21 First, GPs in the Netherlands reported that the proportion of patients with MM increased by 67% over a 20-year period (from 12.3% in 1985 to 20.5% in 2005).²¹ Another Dutch study showed a 28% increase over 7 years (from 12.7% in 2004 to 16.2% in 2011).⁵ The deviation from the Dutch studies can be explained by target population differences between hospital and primary settings, where proportion estimates from the primary health-care sector were among patients with and without chronic diseases.

The large increase in the number of patients undergoing treatment in multiple clinics in our study could be explained by several factors. First, the fact that patients with chronic diseases presumably live longer in 2014 than 2004 due to treatment advances, thereby they have more opportunity to acquire more diseases. Since the severity of the diseases might increase over time or the additional diseases render treatment more complex, the GP refers more often to specialized hospital outpatient care. Second, the Danish health-care system recognized more diseases as a hospital specialty in 2014 than in 2004 (e.g. sleep disorders). Third, the Danish health-care system offered hospital treatment at an earlier stage of the disease in 2014 than in 2004 (e.g. insulin treatment for patients with type 2 diabetes).

Our second finding, the most common pair of specialty clinic combinations, has not previously been studied at the hospital level. A systematic review from primary care reported that hypertension and osteoarthritis were the most frequent combination, followed by different combinations of cardiovascular diseases.²² This shows that MM may consist of many disease combinations in different patterns and are health-care specific, which must be considered when using our research for clinical practice.

Implications

The notable increase over the past 10 years in the number of patients who are treated in multiple hospital outpatient specialty clinics simultaneously gives rise to a number of considerations. While a 7.7% total may seem small, this is probably an underestimation since we did not include all diseases or hospital clinics. Furthermore, we presume that the proportion of the appointments consumed by these patients is at least twice as high. A Danish study on patients from the primary health-care sector showed an additive rise in outpatient appointments with number of diseases; that is, for example, patients with three chronic diseases had nearly three times as many annual outpatient appointments as patients with one chronic disease.²³ As the number of patients with MM is expected to increase, we expect a future increase of patients in multiple hospital outpatient care.

This encourages a focus on treatment regimens that take multiple diseases into account. One way is to identify common disease combinations and address which clinics should benefit from close corporation. For example, we found that patients often attend an endocrine clinic in combination with another type of clinic. Clinical pathways or disease management programs already exist for some combinations of diseases (e.g. diabetes, cardiovascular disease).²⁴ However, this study also found that it is the low-frequency combinations that account for most of the combinations. Future strategies for MM patient management should therefore be generic and non-disease specific. Approaches to multidisciplinary care for people with MM have already been initiated in Denmark and countries with similar health-care systems.²⁵

Strengths and limitations

Our study has some strengths. First, we had a large study sample, covering 100% of all Danish hospitals; second, because of the Danish health-care structure, all of the examined patient groups with specialized needs are referred to hospital outpatient specialty care, meaning that the development reflects a true increase; third, we sampled at three periods; and fourth, we included 50 diagnoses. This latter “strength” could also be seen as a limitation, however, as we did not include all patient groups in hospital treatment for chronic diseases and cannot calculate the total proportion of patients in hospital outpatient treatment for chronic diseases. Our study therefore probably shows an underestimation of the true figure.

Another limitation is that the process of how we chose diagnoses was based on frequent diagnoses at a total hospital level and then categorizing them into organizational outpatient specialty clinics. In that sense, we have not included all hospital specialty clinics in this study; only if the clinic cared for a diagnosis which was within the 50 most frequent hospital diagnoses.

This study did not include psychiatric hospitals, as they are separate from somatic hospitals in the Danish system. While information pertaining to patients treated in both somatic and psychiatric hospitals is important, it is not the focus in this study. Additionally, our study did not show if patients had other, less severe diseases than those investigated in our study. Patients may have one or even several courses of treatment for chronic diseases for which they are receiving treatment from their GP.

The decision about which diseases to include in the study was based on expert opinions. Other experts might consider otherwise. The diagnoses we included are from the NPR and generally of high validation, but not all diagnoses are validated.¹³ We used primary diagnoses set by a specialized doctor in a hospital outpatient specialized clinic and had a 6-month criterion for inclusion, which strengthens the correctness of the diagnoses.

Further limitations include the categorization of diagnoses. We categorized diagnoses into clinical organizational groups using information from typical Danish hospital organizations. However, some hospitals may have organized their outpatient clinics differently.

Our results reflect a health-care system in which the specialist services for MM patients are almost completely covered by the hospital sector. In other countries, some specialists are operating outside the hospitals and our conclusions might not be applicable to such systems.

We did not include factors such as socioeconomic status, which has been shown in other studies to have a strong association with MM⁴ and would have been relevant to adjust for in the time-trend investigation and an analysis of different characteristics between patients in one or several hospital outpatient treatments.

Conclusions

This study has described the development of patients managed simultaneously in multiple hospital outpatient specialty clinics. We found that the proportion has increased nearly 100% over 10 years. Further, we found great diversity in the combination of specialty clinics in which patients with two diseases are treated. Our findings urge hospital outpatient managers to prepare for a future where increasing numbers of MM patients require more complex care. Research is needed on effective and cost-efficient initiatives using integrated care regimens for patients treated simultaneously in multiple outpatient specialty clinics.

Footnotes

Authors’ note

Due to Danish laws on personal data, data cannot be shared publicly. To request these data, please contact the corresponding author for more information.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported in part by a Grant from, Knud og Edith Eriksens Mindefond. The funding source played no role in the study design, the collection, analysis, or in the interpretation of data.

ORCID iD

Donna Lykke Wolff

Appendix 1

Table 1A.

Organizational diagnosis-group, ICD-10 code, and diagnosis.

Specialty clinic	ICD-10 code	Diagnosis
Oncology	DC18	Malignant neoplasm of colon
	DC20	Malignant neoplasm of rectum
	DC34	Malignant neoplasm of bronchus and lung
	DC50	Malignant neoplasm of breast
Dermatology	DC44	Other malignant neoplasms of skin
	DL40	Psoriasis
Urology	DC61	Malignant neoplasm of prostate
Hematology	DC91	Lymphoid leukemia
Endocrinology	DE05	Thyrotoxicosis (hyperthyroidism)
	DE10	Type 1 diabetes mellitus
	DE11	Type 2 diabetes mellitus
	DE66	Obesity
	DM81	Osteoporosis without pathological fracture
Neurology	DG35	Multiple sclerosis
	DG40	Epilepsy
Ophthalmology	DH02	Other disorders of eyelid
	DH25	Senile cataract
	DH35	Other retinal disorders
	DH40	Glaucoma
Ear, nose, and throat	DH90	Conductive and sensorineural hearing loss
	DH91	Other hearing loss
	DH93	Other disorders of ear, not elsewhere classified
	DG47	Sleep disorders
Cardiology	DI10	Essential (primary) hypertension
	DI20	Angina pectoris
	DI25	Chronic ischemic heart disease
	DI35	Nonrheumatic aortic valve disorders
	DI48	Atrial fibrillation and flutter
	DI50	Heart failure
Pulmonary	DJ30	Vasomotor and allergic rhinitis
	DJ44	Other chronic obstructive pulmonary disease
	DJ45	Asthma
Gastroentrology	DK50	Crohn disease (regional enteritis)
	DK51	Ulcerative colitis
	DK58	Irritable bowel syndrome
	DB18	Chronic viral hepatitis
Rheumatology	DM05	Seropositive rheumatoid arthritis
	DM06	Other rheumatoid arthritis
	DM07	Psoriatric and enteropathic arthropathies
	DM45	Spondylitis ankylopoietica
Osteoarthrosis	DM16	Coxarthrosis (arthrosis of hip)
	DM17	Gonarthrosis (arthrosis of knee)
	DM23	Internal derangement of knee
	DM48	Other spondylopathies
	DM75	Shoulder lesions
	DM79	Other soft tissue disorders, not elsewhere classified
Back	DM51	Other intervertebral disc disorders
	DM54	Dorsalgia
Nephrology	DN18	Chronic kidney disease
Vascular surgery	DI73	Other peripheral vascular diseases

Appendix 2

Table 2B.

Number and relative distribution of treatment courses by diagnosis in 2004, 2009, and 2014.

Specialty clinic	ICD	Diagnosis	2004		2009		2014
			Treatment courses		Treatment courses		Treatment courses
			n	%	n	%	n	%
Total			185,143^a	100	262,520^a	100	372,976^a	100
Oncology	DC18	Malignant neoplasm of colon	902	0.5	1759	0.7	3028	0.8
	DC20	Malignant neoplasm of rectum	660	0.4	932	0.4	1533	0.4
	DC34	Malignant neoplasm of bronchus and lung	981	0.5	1561	0.6	2942	0.8
	DC50	Malignant neoplasm of breast	12,654	6.8	16,382	6.2	25,903	6.9
		Group total	15,197	8.2	20,634	7.9	33,406	9.0
Dermatology	DC44	Other malignant neoplasms of skin	1331	0.7	2169	0.8	3014	0.8
	DL40	Psoriasis	1201	0.6	1901	0.7	3660	1.0
		Group total	2532	1.4	4070	1.6	6674	1.8
Urology	DC61	Malignant neoplasm of prostate	3826	2.1	8950	3.4	12974	3.5
Urology		Group total	3826	2.1	8950	3.4	12,974	3.5
Hematology	DC91	Lymphoid leukemia	1785	1.0	2336	0.9	3333	0.9
Hematology		Group total	1785	1.0	2336	0.9	3333	0.9
Endocrinology	DE05	Thyrotoxicosis (hyperthyroidism)	6944	3.8	6916	2.6	7416	2.0
	DE10	Type 1 diabetes mellitus	16,859	9.1	18,462	7.0	20,308	5.4
	DE11	Type 2 diabetes mellitus	17,030	9.2	24,870	9.5	28,666	7.7
	DE66	Obesity	962	0.5	2867	1.1	3392	0.9
	DM81	Osteoporosis without pathological fracture	2850	1.5	6788	2.6	12,926	3.5
		Group total	44,645	24.1	59,903	22.8	72,708	19.5
Neurology	DG35	Multiple sclerosis	5169	2.8	7675	2.9	9657	2.6
	DG40	Epilepsy	11412	6.2	13037	5.0	15165	4.1
		Group total	16,581	9.0	20,712	7.9	24,822	6.7
Ophthalmology	DH02	Other disorders of eyelid	477	0.3	1075	0.4	1854	0.5
	DH25	Senile cataract	1406	0.8	2317	0.9	3472	0.9
	DH35	Other retinal disorders	1220	0.7	4394	1.7	8734	2.3
	DH40	Glaucoma	828	0.4	1111	0.4	1583	0.4
		Group total	3931	2.1	8897	3.4	15,643	4.2
Ear, nose, and throat	DH90	Conductive and sensorineural hearing loss	1783	1.0	2231	0.8	3554	1.0
	DH91	Other hearing loss	8877	4.8	10931	4.2	13519	3.6
	DH93	Other disorders of ear, not elsewhere classified	1632	0.9	2366	0.9	3198	0.9
	DG47	Sleep disorders	4493	2.4	9431	3.6	18946	5.1
		Group total	16,785	9.1	24,959	9.5	39,217	10.5
Cardiology	DI10	Essential (primary) hypertension	4842	2.6	5600	2.1	7060	1.9
	DI20	Angina pectoris	3190	1.7	3730	1.4	4281	1.1
	DI25	Chronic ischemic heart disease	3628	2.0	4835	1.8	6770	1.8
	DI35	Nonrheumatic aortic valve disorders	3449	1.9	7248	2.8	12,986	3.5
	DI48	Atrial fibrillation and flutter	5900	3.2	8839	3.4	14,562	3.9
	DI50	Heart failure	2524	1.4	4740	1.8	7183	1.9
		Group total	23,533	12.7	34,992	13.3	52,842	14.2
Pulmonary	DJ30	Vasomotor and allergic rhinitis	990	0.5	900	0.3	1308	0.4
	DJ44	Other chronic obstructive pulmonary disease	3686	2.0	6745	2.6	9814	2.6
	DJ45	Asthma	3744	2.0	4785	1.8	7858	2.1
		Group total	8,420	4.5	12,430	4.7	18,980	5.1
Gastroenterology	DK50	Crohn disease (regional enteritis)	5036	2.7	6355	2.4	8636	2.3
	DK51	Ulcerative colitis	8600	4.6	11039	4.2	14,999	4.0
	DK58	Irritable bowel syndrome	614	0.3	926	0.4	1466	0.4
	DB18	Chronic viral hepatitis	1857	1.0	3011	1.1	5103	1.4
		Group total	16,107	8.7	21,331	8.1	30,204	8.1
Rheumatology	DM05	Seropositive rheumatoid arthritis	7143	3.9	9655	3.7	12,832	3.4
	DM06	Other rheumatoid arthritis	3391	1.8	4741	1.8	6468	1.7
	DM07	Psoriatric and enteropathic arthropathies	1602	0.9	3283	1.3	5317	1.4
	DM45	Spondylitis ankylopoietica	936	0.5	1818	0.7	2653	0.7
		Group total	13,072	7.1	19,497	7.4	27,270	7.3
Osteoarthrosis	DM16	Coxarthrosis (arthrosis of hip)	4173	2.3	4267	1.6	3294	0.9
	DM17	Gonarthrosis (arthrosis of knee)	3046	1.6	4065	1.5	4749	1.3
	DM23	Internal derangement of knee	1393	0.8	1334	0.5	3044	0.8
	DM48	Other spondylopathies	956	0.5	1044	0.4	1539	0.4
	DM75	Shoulder lesions	1400	0.8	2213	0.8	3584	1.0
	DM79	Other soft tissue disorders, not elsewhere classified	653	0.4	789	0.3	1538	0.4
		Group total	11,621	6.3	13,712	5.2	17,748	4.8
Back	DM51	Other intervertebral disc disorders	1960	1.1	1916	0.7	2226	0.6
	DM54	Dorsalgia	1410	0.8	1465	0.6	2052	0.6
		Group total	3370	1.8	3381	1.3	4278	1.1
Nephrology	DN18	Chronic kidney disease	2523	1.4	4523	1.7	9882	2.6
Nephrology		Group total	2523	1.4	4523	1.7	9882	2.6
Vascular surgery	DI73	Other peripheral vascular diseases	1215	0.7	2193	0.8	2995	0.8
Vascular surgery		Group total	1215	0.7	2193	0.8	2995	0.8

^aThe total number of treatment courses is higher than the total number of patients, as shown in Table 1, as patients with several treatment courses occur under more than one diagnosis. Furthermore, patients can be enrolled with more than one diagnoses within each specialty clinic.

References

Mishra

Jones

. Evidence on multimorbidity from definition to intervention: an overview of systematic reviews. Ageing Res Rev 2017; 37: 53–68.

Willadsen

Bebe

Koster-Rasmussen

. The role of diseases, risk factors and symptoms in the definition of multimorbidity - a systematic review. Scand J Prim Health Care 2016; 34(2): 1–10.

The Danish Health Authority. The Danish national health profile 2017 [Danskernes sundhed - den nationale sundhedsprofil 2017]. Copenhagen: The Danish Health Authority, 2018 [in Danish].

Barnett

Mercer

Norbury

. Epidemiology of multimorbidity and implications for health care, research, and medical education: a cross-sectional study. Lancet 2012; 380: 37–43.

van Oostrom

Gijsen

Stirbu

. Time trends in prevalence of chronic diseases and multimorbidity not only due to aging: data from general practices and health surveys. PLoS One 2016; 11: e0160264.

Partnership for Solutions. Chronic conditions: making the case for ongoing care. Baltimore: The Johns Hopkins University, 2004.

Kingston

Robinson

Booth

. Projections of multi-morbidity in the older population in England to 2035: estimates from the Population Ageing and Care Simulation (PACSim) model. Age Ageing 2018; 47: 374–380.

Schiotz

Host

Frolich

. Involving patients with multimorbidity in service planning: perspectives on continuity and care coordination. J Comorb 2016; 6: 95–102.

Statistics Denmark. Population count [Folketal 2018], https://www.dst.dk/da/Statistik/emner/befolkning-og-valg/befolkning-og-befolkningsfremskrivning/folketal. (2018, accessed 29 September 2018 [in Danish]).

10.

Pedersen

Andersen

Sondergaard

. General practice and primary health care in Denmark. J Am Board Fam Med 2012; 25(Suppl 1): S34–S38.

11.

The Ministry of Health. Healthcare in Denmark: an overview, https://www.sum.dk/∼/media/Filer%20-%20Publikationer_i_pdf/2016/Healthcare-in-dk-16-dec/Healthcare-english-V16-dec.pdf. (2016, accessed 29 September 2018).

12.

Lynge

Sandegaard

Rebolj

. The Danish National Patient Register. Scand J Public Health 2011; 39: 30–33.

13.

Schmidt

Sandegaard

. The Danish National Patient Registry: a review of content, data quality, and research potential. Clinical epidemiology 2015; 7: 449–490.

14.

Diederichs

Berger

Bartels

. The measurement of multiple chronic diseases–a systematic review on existing multimorbidity indices. J Gerontol A Biol Sci Med Sci 2011; 66: 301–311.

15.

Fortin

Stewart

Poitras

. A systematic review of prevalence studies on multimorbidity: toward a more uniform methodology. Ann Fam Med 2012; 10: 142–151.

16.

The Danish Health Data Authority. Hospital outpatient activity on diagnosis level [Aktivitet på diagnoseniveau], http://www.esundhed.dk/sundhedsregistre/LPR/Sider/LPR01_Tabel.aspx (2018, accessed 2 November 2018 [in Danish], accessed 18 November 2018).

17.

The Danish Health Authority. Criteria for the preparation of treatment programs for chronic diseases [Kriterier for udarbejdelse af forløbsprogrammer for kroniske sygdomme]. 2011 [in Danish].

18.

Statistics Denmark. Classification of diagnoses based on organization of hospital care [DIAG23 sygehusafdelingers organisering af behandlingen], http://www.dst.dk/da/Statistik/dokumentation/Times/sygehusbenyttelse/diag23 (2018, accessed November 8 2018 [in Danish], accessed 1 March 2017).

19.

The Regional Committees on Health Research Ethics for Southern Denmark. Research trials that do not require approval [Forsøgstyper uden anmeldelsespligt], https://komite.regionsyddanmark.dk/wm428123 (2018, accessed 29 September 2018 [in Danish]).

20.

Von Elm

Altman

Egger

. Strengthening the reporting of observational studies in epidemiology (STROBE) statement: guidelines for reporting observational studies. BMJ 2007; 335: 806–808.

21.

Uijen

van de Lisdonk

. Multimorbidity in primary care: prevalence and trend over the last 20 years. Eur J Gen Pract 2008; 14(Suppl 1): 28–32.

22.

Violan

Foguet-Boreu

Flores-Mateo

. Prevalence, determinants and patterns of multimorbidity in primary care: a systematic review of observational studies. PLoS One 2014; 9: e102149.

23.

Statens Serum Institut. Store udgifter forbundet med multisygdom. 2015. Copenhagen.

24.

Chamberlain

Johnson

Leal

. Cardiovascular disease and risk management: review of the American Diabetes Association Standards of Medical Care in Diabetes 2018. Ann Intern Med 2018; 168: 640–650.

25.

Rijken

Hujala

van Ginneken

. Managing multimorbidity: profiles of integrated care approaches targeting people with multiple chronic conditions in Europe. Health Policy 2013; 122: 44–52.