Analysis of chlamydia contacts and the look-back interval at a UK sexual health clinic

Abstract

Guidance on contact tracing in Chlamydia trachomatis (CT) is limited. CT contacts data over 12 months (1 December 2018–29 November 2019) at a UK sexual health clinic were analysed to determine the appropriateness of the currently recommended six-month ‘look-back’ interval. Age and sex of CT contacts were associated with clinical outcomes. Subgroups of 100 CT positive/negative contacts (each N = 100) were randomly selected. The relationship between time since sexual intercourse with the index case (Last Sexual Intercourse; LSI) and CT positivity was examined; suitability of varying look-back intervals was explored. Of 891 CT contacts (mean age = 25.0 years, 66.2% men), 66.9% tested positive for CT. Positive CT contacts were significantly younger (23.8 ± 6.8 years vs. 27.4 ± 9.1, p < 0.001) and more often women (36.4% vs. 28.5%, p = 0.018) than negative contacts. In the subgroups, the Mann–Whitney U test revealed no significant difference between the LSI of positive and negative contacts (p = 0.081). 95% of positive CT contacts (N = 82) were captured within a hypothetical three-month look-back interval. While most CT positive contacts were captured within three months, they appeared to remain proportionately represented beyond this point. Although this supports current guidelines, further research should investigate whether CT contacts involved in longer look-back intervals may require disproportionately greater resources to trace.

Keywords

Chlamydia contact look-back interval tracing

Introduction

Chlamydia trachomatis (CT) is the most commonly diagnosed sexually transmitted infection (STI) in England. In 2019, there were 468,342 new STI diagnoses made at sexual health services (SHS) in England, with almost half (229,411; 49%) being attributed to CT.¹ Chlamydia trachomatis infection can be asymptomatic meaning patients may not present for testing and treatment. Identifying CT at an early stage is particularly important because untreated infection can increase the risk of pelvic inflammatory disease, infertility and ectopic pregnancies.² Current guidance suggests that anyone under the age of 25 years who is sexually active should be screened for CT annually and on change of sexual partner;³ more frequent screening is recommended for men who have sex with men (MSM) and varies depending on behaviour.⁴

When a patient tests positive for CT, a healthcare professional such as a health advisor looks back over a period of time (the ‘look-back interval’) to identify sexual partners of the index case who require testing and treating – these patients are referred to as CT contacts, and this process is referred to as partner notification (PN). There is little research describing the effectiveness of specific look-back intervals, and current guidelines are recommendations only. British Association for Sexual Health and HIV (BASHH) guidelines suggest a look-back interval of four weeks for symptomatic men and six months for all other CT-positive patients (or most recent partner if this is greater than six months⁵). All partners identified within the look-back interval should be given empirical antibiotic treatment at the time of testing (7 days of doxycycline or equivalent⁵).

There is a need for a well-defined, evidence-based look-back interval in chlamydial infection. At this UK city centre SHS, a look-back interval of six months is used for all patients who test positive for CT. Contacts are initially identified following discussion with the index case and then reached out to via phone call, text messaging or post depending on available information. A look-back interval of fewer than six months, if safe, could reduce costs (1,339,931 CT tests in 15–24 year olds were taken in 2019¹ with an approximate cost of £45 per screening test⁶); reduce workload (one study found the average time to trace a CT-positive contact was 6.8 h⁷) and reduce empirical antibiotic treatment. Decreased antibiotic use will not only reduce medication side effects such as hypersensitivity, gastrointestinal issues and photosensitivity⁸ and lower antibiotic resistance but may also reduce the risks associated with gut microbial dysbiosis.⁹ Conversely, a shorter look-back interval risks missing a number of positive CT contacts which in turn may increase the risk of further onwards transmission and long-term chlamydial complications.

This study aims to examine the relative effectiveness of a hypothetical shorter look-back interval in identifying positive CT contacts than the currently used six-month interval. In addition, this study will determine the prevalence of empirical antibiotic use and highlight the risk factors documented in CT contacts.

Methods

Participants

An automatic search of the electronic patient records system at a UK sexual health clinic was undertaken using Microsoft Access to identify all chlamydia contacts (GUMCAD; SHHAPT¹⁰ code ‘PNC’ and local code ‘PNCM’ (treated chlamydia contact)) between 1 December 2018 and 29 November 2019 (a one-year period). The Microsoft Access query ensured patient demographics (age/sex/ethnicity), hospital identification number, patient risk factors (none/MSM/intravenous drug user (IVDU)/commercial sex worker (CSW)/not known), SHHAPT chlamydia infection site codes¹⁰ and the dates of CT screening and treatment were automatically generated data alongside the PNC/PNCM codes. These data were transferred into an Excel spreadsheet for further analysis. The project was registered with the Clinical Effectiveness Unit (within Sheffield Teaching Hospitals NHS Foundation Trust) and was conducted according to the principles of the World Medical Association Declaration of Helsinki. This was a retrospective analysis of already acquired clinical data; patient information was fully anonymised, and therefore, consent was not obtained.

Data retrieval methods

A random number generator was used to generate a sample of 200 CT contacts for analysis; 100 negative and 100 positive CT contacts from the 891 total contacts.¹¹ This was possible by searching for the SHHAPT code for CT-positive patients ‘C4’ ‘C4R’ ‘C4X’ ‘C4O’ ‘C4RX’ ‘C4OX’ (which code for CT infection at genital, pharyngeal and rectal sites) within the spreadsheet of all CT contacts. Chlamydia trachomatis negative contacts did not exhibit any of these codes.

The local Health Advisor Partner Notification Database (HAPND) was primarily used to retrieve additional information about each of these 200 CT contacts and the CT positive index case associated with each contact. By searching for each CT contact manually within the HAPND, the following key details were gathered:

1. The number of days from last sexual intercourse (LSI) between the contact and the index case up until the date the CT contact receives treatment (LSI as reported by index case unless unavailable in which case LSI information sought from CT contact electronic patient notes).

2. The relationship status of the CT contact (as reported by the index case) – the relationship status was recorded as regular or non-regular (ex or on/off).

Due to missing or duplicate data, there were 100 positive contacts and 95 negative contacts in the final samples for analyses.

Statistical analysis

All statistics were performed in SPSS version 25 (IMB Corp.)

Demographic differences between CT contacts who tested positive or negative were statistically examined; X² analysis tested for sex differences, while independent t-test tested for age differences. Further analyses then focused on the final study subgroups. X² analysis was used to look at whether positive or negative contacts were more likely to have a regular partner. The Mann–Whitney U test was used to analyse whether any differences in mean LSI (between CT contact and index case) existed between positive and negative CT contact subgroups. Last sexual intercourse day cut-offs were determined where 90%, 95% and 99% of positive contacts were captured; the relative loss of both positive and negative contacts beyond each of these cut-offs was recorded. Finally, patient demographics and CT positivity rates in 0–3 and >3 months look-back interval groups were summarised.

Results

892 CT contacts were identified; one patient was excluded as their age was incorrectly recorded as 0.2 years old, leaving 891 total CT contacts (age range 14–78 years old). The CT contacts had a mean age of 25.0 years and were 66.2% men (N = 590) and 33.8% women (N = 301). They displayed a diverse ethnic profile: white British/Irish (N = 506, 56.8%), African/African Caribbean (N = 65, 7.3%), Asian (N = 41, 4.6%), other ethnicity (N = 83, 9.3%) and no ethnicity reported (N = 196, 22.0%). 591 (66.3%) patients were documented as having ‘no risk’ identified, 43 (4.8%) were MSM, zero were current IVDU, one (0.1%) was a CSW and 19 (2.1%) were documented as risk ‘not known’. In total, 659 (74.0%) had a level of risk documented, leaving 232 (26.0%) without inputted data.

Of all CT contacts at this UK sexual health service over a one-year period, 596/891 (66.9%) tested positive for CT. Women accounted for significantly more of the CT-positive patients (36.4%) than they did of CT-negative patients (28.5%)(p = 0.018). CT-positive contacts were also significantly younger (mean age = 23.8 years, SD = 6.8) than the CT-negative contacts (mean age = 27.4 years, SD = 9.1, p < 0.001).

After excluding missing data, there were 100 positive CT contacts and 95 negative CT contacts in the final groups for analysis. 57.0% (N = 57) of the positive CT contacts and 56.8% (N = 54) of the negative CT contacts had a regular partner, and no significant difference was found (p = 0.982).

In the positive CT contact group, 8/100 patients had no data recorded about whether they had received treatment. In the negative CT contact group, only 1/95 was not treated (reason not recorded), and 1/95 had no data. In total, 92/100 positive contacts and 93/95 negative contacts were documented as having received antibiotic treatment.

The LSI information was available for 86 positive and 90 negative CT contacts. Last sexual intercourse was non-normally distributed. Median (SD) LSI was 14.0 (29.1) and 17.5 (30.4) days for positive and negative contacts, respectively; there was no significant difference in LSI between these two groups (p = 0.081). To capture 90% of positive CT contacts (77 patients to the nearest whole number), it was necessary to look back a period of 43 days. To capture 95% of positive CT contacts (N = 82), it was necessary to look back a period of 68 days, and to capture 99% (N = 85), it was necessary to look back a period of 143 days Figure 1 visualises this information. Using a three-month look-back period (i.e. 90 days) as a hypothetical alternative cut-off, three contacts in each of the positive and negative subgroups would have been missed/excluded. Table 1 shows the similar chlamydia positivity rates in the 0–3 and >3-month look-back intervals.

Figure 1.

Line graph showing how the percentage of positive CT contacts relates to the LSI with the index case. Note: CT: Chlamydia trachomatis; LSI: last sexual intercourse.

Table 1.

An overview of key variables between chlamydia contact subgroups with last sexual intercourse of less than and greater than three months.

Look-back interval	Age (SD)(years)	Sex (% male)	Chlamydia positivity
0–3 months (N = 170)	25.2 ± 7.9	73.5% (N = 125)	48.8% (N = 83)
>3 months (N = 6)	21.2 ± 4.3	33.3% (N = 2)	50.0% (N = 3)

Discussion

Approximately two thirds of all CT contacts over the period 1 December 2018–29 November 2019 tested positive for CT. This finding is in keeping with previous studies which have found that 63.7%¹² and 62%¹³ of CT contacts were positive. This study also shows that women are statistically more likely than men to be positive CT contacts at this sexual health service. Interestingly, a systematic review conducted in 2013 found that in nine of 25 studies, women had a higher prevalence of CT than men and concluded that a modest sex difference was apparent.¹⁴ The literature indicates that women are more likely to experience symptomatic chlamydia infection;^14,15 this may provide additional motivation for them to engage in PN and attend clinic as a chlamydia-positive contact. In addition, this study supports recent chlamydia screening figures in England¹ in showing that younger patients are more likely to be positive for CT.

Previous research acknowledges that there is a balance needed with regards to workload in contact tracing to make it an efficient process.¹⁶ Interestingly, the date of LSI does not appear to be an indicator of whether a CT contact is more or less likely to test positive for CT. In this study, 95% of positive CT contacts would have been captured within 68 days; however, it appears that a longer look-back period still serves a purpose in identifying positive contacts and at a rate which remains in relative proportion to negative contacts. In fact, the overall rates of chlamydia positivity were highly comparable in both the 0–3 and >3-month look-back interval groups, each approximately 50%. This is far greater than any estimated general population of chlamydia prevalence,¹⁴ therefore, demonstrating a continuing utility of an extended look-back interval. This study also implies minimal extra overall workload beyond three months (only 3% of all contacts were found beyond 90 days in this study). However, anecdotes from health advisors suggest that this finding may not be an accurate representation of the workload involved in contact tracing and that a significant proportion of their time focuses on the three- to six-month look-back interval. Future studies require data about the workload involved in PN and patient engagement; LSI may affect this, for example, longer LSI duration may involve more laborious contact tracing work/greater loss to follow-up, or shorter LSI duration may increase the likelihood of the patient self-referring before the health advisor contacts them. Establishing the effect of LSI duration on workload is important as this will indicate if more resources are needed in the PN system. Therefore, although this current analysis demonstrates that CT contacts who are successfully identified do benefit from a six-month look-back period, it is still possible that achieving this requires a disproportionate workload compared to shorter periods.

Of interest, in both the CT-positive and CT-negative contact groups, there was one patient whose LSI was outside of the six-month period (184 and 219 days, respectively). Given current guidelines, these patients were captured due to this being their most recent sexual partner, and this highlights the importance of asking CT-positive patients about their most recent sexual partner if this is outside of the allotted six months.

In this study, approximately one third of all CT contacts were negative and therefore received antibiotics unnecessarily. Empirical antibiotic use is important in CT contacts not only because asymptomatic infection is common but also because patients can become lost to follow-up. Additionally, one study found it took a mean of 1.9 days for patients to attend clinic for CT treatment after notification of positive results.¹⁷ Minimising the use of antibiotics, however, is also important in order to reduce side effects and interactions with medications as well as cut costs. Although antibiotic resistance to CT remains rare, with no known resistance to doxycycline,¹⁸ emerging antibiotic resistance is a growing concern and has guided CT treatment advice in recent years.^19,20 Further research could focus on whether it would be feasible not to always treat patients empirically with antibiotics until CT status is known. To consider this prospect, work would need to be done on improving patient engagement with SHS to reduce the rates of patients lost to follow-up and improving the turnaround time of CT testing kits – if same-day results were available (and patients were outside of the CT window period), empirical antibiotic use and issues surrounding loss to follow-up could fall substantially.

Sexual health service patients should have their risks recorded clearly and accurately. At this SHS, 26.0% did not have their risks documented on the HAPND. This information may be particularly useful in helping to decide specific look-back intervals based on patient risk. Perhaps a longer look-back interval is warranted in higher risk patients – further research is needed to address this.

There were some limitations with this study. Firstly, it is important to note that the results in this study are reliant on CT contacts and CT status being coded correctly in the database. Whilst conducting the study, it became apparent that some patients were simply coded as chlamydia-positive ‘C4’ when in fact, a more accurate chlamydia infection site code could have been utilised such as ‘C4R’ (rectal) or ‘C4O’ (oropharyngeal). Secondly, there is a lack of data pertaining to PN workload in this study thereby meaning it is difficult to weigh up the workload burden against the identification of chlamydia-positive contacts in the longer look-back period. Lastly, the results of this study are of most relevance to sexual health services in the United Kingdom which are most likely to have a similar PN process and target audience. Of note, patients captured in this study are not representative of contacts who were lost to follow-up, those who were followed up at other services or those who obtained online testing and treatment. Importantly, PN pathways are continuing to evolve – internet tests increased by 22% between 2018 and 2019,¹ and since the coronavirus pandemic, postal testing and treatment has increased in popularity – it is possible that the remote nature of these pathways may exacerbate the PN challenges associated with tracing patients; however, face-to-face workload is likely to decrease.

This study examined the efficacy of the current six-month look-back interval in CT contacts as well as documenting both empirical antibiotic use and patient risk factors in this cohort. Although this study continues to support current guidelines, larger scale studies investigating the PN workload, costs and CT positivity rates in differing look-back intervals are needed in order to suggest shortening the look-back interval.

Footnotes

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) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Catherine Turner

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