Multiple measures of adherence have been reported in the research literature and it is difficult to determine which is best, as each is nuanced. Occurrences of medication switching and polypharmacy or therapeutic duplication can substantially complicate adherence calculations when adherence to a therapeutic class is sought.
OBJECTIVE
To contrast the Proportion of Days Covered (PDC) adherence metric with 2 variants of the Medication Possession Ratio (MPR, truncated MPR).
METHODS
This study was a retrospective analysis of the North Carolina Medicaid administrative claims data from July 1999 to June 2000. Data for patients with schizophrenia (ICD-9-CM code 295.xx) who were not part of a health maintenance organization, not hospitalized, and not pregnant, taking at least one antipsychotic, were aggregated for each person into person-quarters. The numerator for PDC was defined as the number of days one or more antipsychotics was available and the MPR numerator was defined as the total days’ supply of antipsychotics; both were divided by the total days in each person-quarter. Adherence rates were estimated for subjects who used only one antipsychotic, switched medications, or had therapeutic duplication in the quarter.
RESULTS
The final sample consisted of 25,200 person-quarters from 7069 individuals. For person-quarters with single antipsychotic use, adherence to antipsychotics as a class was: PDC 0.607, truncated MPR 0.640, and MPR 0.695 (p < 0.001). For person-quarters with switching, the average MPR was 0.690, truncated MPR was 0.624, and PDC was 0.562 (p < 0.001). In the presence of therapeutic duplication, the PDC was 0.669, truncated MPR was 0.774, and MPR was 1.238 (p < 0.001).
CONCLUSIONS
The PDC provides a more conservative estimate of adherence than the MPR across all types of users; however, the differences between the 2 methods are more substantial for persons switching therapy and prescribed therapeutic duplication, where MPR may overstate true adherence. The PDC should be considered when a measure of adherence to a class of medications is sought, particularly in clinical situations in which multiple medications within a class are often used concurrently.
HearnshawH, LindenmeyerA. What do we mean by adherence to treatment and advice for living with diabetes? A review of the literature on definitions and measurements. Diabet Med2006; 23: 720–8.
2.
VelliganD, LamY, GlahnD, Defining and assessing adherence to oral antipsychotics: a review of the literature. Schizophr Bull2006; 32: 724–42. Epub 17 May 2006.
3.
HaynesR, TaylorD, SackettD, eds. Compliance in health care. Baltimore: Johns Hopkins University Press, 1979.
4.
SackettD, HaynesR, eds. Compliance with therapeutic regimens. Baltimore: John Hopkins University Press, 1976.
5.
SchedlbauerA, SchroederK, PetersT, FaheyT. Interventions to improve adherence to lipid lowering medication. Cochrane Database Syst Rev2004; 18(4):CD004371.
6.
SackettD, HaynesR, GibsonE, Randomized clinical trial of strategies for improving medication compliance in primary hypertension. Lancet1975; 1: 1205–7.
7.
SackettD, SnowJ, eds. The magnitude of compliance and noncompliance. Baltimore: Johns Hopkins Univ Press, 1979.
8.
VermeireE, HearnshawH, Van RoyenP, DenekensJ. Patient adherence to treatment: three decades of research. A comprehensive review. J Clin Pharm Ther2001; 26: 331–42.
9.
HessLM, RaebelMA, ConnerDA, MaloneDC. Measurement of adherence in pharmacy administrative databases: a proposal for standard definitions and preferred measures. Ann Pharmacother2006; 40: 1280–8. Epub 25 Jul 2006. DOI 10.1345/aph.1H018
10.
EllisS, ShumakerS, SieberW, RandC, Pharmacological Intervention Working Group. Adherence to pharmacological interventions: current trends and future directions. Control Clin Trials2000; 21 (5 suppl): 218S–25S.
11.
AndradeSE, KahlerKH, FrechF, ChanKA. Methods for evaluation of medication adherence and persistence using automated databases. Pharmacoepidemiol Drug Saf2006; 15: 565–74.
12.
ChooP, RandC, InuiT, Validation of patient reports, automated pharmacy records, and pill counts with electronic monitoring of adherence to antihypertensive therapy. Med Care1999; 37: 846–57.
13.
SteinerJF, KoepsellTD, FihnSD, InuiTS. A general method of compliance assessment using centralized pharmacy records. Med Care1988; 26: 814–23.
14.
HamiltonR, BricelandL. Use of prescription-refill records to assess patient compliance. Am J Hosp Pharm1992; 49: 1691–6.
15.
MonaneM, BohnR, GurwitzJ, GlynnR, LevinR, AvornJ. The effects of initial drug choice and comorbidity on antihypertensive therapy compliance: results from a population-based study in the elderly. Am J Hypertens1997; 10: 697–704.
16.
SclarD, ChinA, SkaerT, OkamotoM, NakahiroR, GillM. Effect of health education in promoting prescription refill compliance among patients with hypertension. Clin Ther1991; 13: 489–95.
17.
SclarD, TessierG, SkaerT, LeggR, RobisonL, NemecN. Effect of pharmaceutical formulation of diltiazem on the utilization of Medicaid and health maintenance organization services. Curr Ther Res1993; 55: 1136–49.
18.
StroupeK, TealE, TuW, WeinerM, MurrayM. Association of refill adherence and health care use among adults with hypertension in an urban health care system. Pharmacotherapy2006; 26: 779–89.
19.
SikkaR, XiaF, AubertR. Estimating medication persistency using administrative claims data. American J Manag Care2005; 11: 449–57.
20.
CantrellCR, EaddyMT, ShahMB, ReganTS, SokolMC. Methods for evaluating patient adherence to antidepressant therapy: a real-world comparison of adherence and economic outcomes. Med Care2006; 44: 300–3.
21.
KeeneMS, EaddyMT, NelsonWW, SarnesMW. Adherence to paroxetine CR compared with paroxetine IR in a Medicare-eligible population with anxiety disorders. Am J Manag Care2005; 11 (12 suppl): S362–9.
22.
KarveS, MartinBC. The predictive validity of different adherence measures using administrative claims data. Value Health2007; 10(3): A85.
23.
KarveS, ClevesMA, HelmM, HudsonTJ, WestDS, MartinBC. An empirical basis for standardizing adherence measures derived from administrative claims data among diabetic patients. Med Care2008; 46: 1125–33.
24.
BramleyT, GerbinoP, NightingaleBN, Frech-TamasF. Relationship of blood pressure control to adherence with antihypertensive monotherapy in 13 managed care organizations. J Manag Care Pharm2006; 12: 239–45.
25.
BennerJS, GlynnRJ, MogunH, Long-term persistence in use of statin therapy in elderly patients. JAMA2002; 288: 455–6.
26.
PetersonAM, NauDP, CramerJA, BennerJ, Gwadry-SridharF, NicholM. A checklist for medication compliance and persistence studies using retrospective databases. Value Health2007; 10: 3–12.
27.
DoroP, BenkoR, KosikE, MatuzM, TothK, SoosG. Utilization of oral antihyperglycemic drugs over a 7-year period (1998–2004) in a Hungarian population and adherence to drug therapy. Eur J Clin Pharmacol2005; 61: 893–7.
28.
SajatovicM, ValensteinM, BlowFC, GanoczyD, IgnacioRV. Treatment adherence with antipsychotic medications in bipolar disorder. Bipolar Disorders2006; 8: 232–41.
29.
ChakosM, GlickI, MillerA, Baseline use of concomitant psychotropic medications to treat schizophrenia in the CATIE trial. Psychiatric Serv2006; 57: 1094–101.
30.
GangulyR, KotzanJ, MillerL, KennedyK, MartinB. Prevalence, trends, and factors associated with antipsychotic polypharmacy among Medicaid-eligible schizophrenia patients, 1998–2000. J Clin Psychiatry2004; 65: 1377–88.
31.
KreyenbuhlJ, ValensteinM, McCarthyJ, GanoczyD, BlowF. Long-term combination antipsychotic treatment in VA patients with schizophrenia. Schizophr Res2006; 84: 90–9.
32.
LindenM, PyrkoschL, DittmannR, CzekallaJ. Why do physicians switch from one antipsychotic agent to another? The “physician drug stereotype.”J Clin Psychopharmacol2006;26: 225–31.
33.
McEvoyJ, LiebermanJ, StroupT, Effectiveness of clozapine versus olanzapine, quetiapine, and risperidone in patients with chronic schizophrenia who did not respond to prior atypical antipsychotic treatment. Am J Psychiatry2006; 163: 600–10.
34.
ParepallyH, ChakravortyS, LevineJ, The use of concomitant medications in psychiatric inpatients treated with either olanzapine or other antipsychotic agents: a naturalistic study at a state psychiatric hospital. Prog Neuropsychopharmacol Biol Psychiatry2002; 26: 437–40.
35.
Ascher-SvanumH, FariesD, ZhuB, ErnstF, SwartzM, SwansonJ. Medication adherence and long-term functional outcomes in the treatment of schizophrenia in usual care. J Clin Psychiatry2006; 67: 453–60.
36.
GilmerT, DolderC, LacroJ, Adherence to treatment with antipsychotic medication and health care costs among medicaid beneficiaries with schizophrenia. Am J Psychiatry2004; 161: 692–9.
37.
KatonW, CantrellCR, SokolMC, ChiaoE, GdovinJM. Impact of antidepressant drug adherence on comorbid medication use and resource utilization. Arch Intern Med2005; 165: 2497–503.
38.
ChanK, WalkerA, YoodR. An equilibrium model of drug utilization. J Clin Epidemiol1993; 46: 113–21.
39.
JaffeA, LevineJ. Antipsychotic medication coprescribing in a large state hospital system. Pharmacoepidemiol Drug Saf2003; 12: 41–8.
40.
GlickI. Understanding the results of CATIE in the context of the field. CNS Spectr2006; 11 (7 suppl 7): 40–7.
41.
LiebermanJ. Effectiveness of antipsychotic drugs in patients with chronic schizophrenia: efficacy, safety and cost outcomes of CATIE and other trials. J Clin Psychiatry2007; 68: e04.
42.
LiebermanJ, StroupT, McEvoyJ, Effectiveness of antipsychotic drugs in patients with chronic schizophrenia. N Engl J Med2005; 353: 1209–23.
43.
DominoM, SwartzM. Who are the new users of antipsychotic medications?Psychiatr Serv2008; 59: 507–14.
44.
PreskornS. Pharmacogenomics, informatics, and individual drug therapy in psychiatry: past, present and future. J Psychopharmacol2006; 20 (4 suppl): 85–94.
45.
American Psychiatric Association. Practice guideline for the treatment of patients with schizophrenia. 2nd ed.Arlington, VA: American Psychiatric Association, 2004.
46.
LehmanA, LiebermanJ, DixonL, Practice guideline for the treatment of patients with schizophrenia, second edition. Am J Psychiatry2004; 161 (2 Suppl): 1–56.
47.
CramerJA, RosenheckR. Compliance with medication regimens for mental and physical disorders. Psychiatr Serv1998; 49: 196–201.
48.
ValensteinM, GanoczyD, McCarthyJ, Myra KimH, LeeT, BlowF. Antipsychotic adherence over time among patients receiving treatment for schizophrenia: a retrospective review. J Clin Psychiatry2006; 67: 1542–50.
49.
ValensteinM, CopelandL, BlowF, Pharmacy data identify poorly adherent patients with schizophrenia at increased risk for admission. Med Care2002; 40: 627–9.
