Table 1.

Summary of methods for assessing adherence.

Method	Advantages	Disadvantages	Direction of potential bias	Comments
Physician’s assessment	Simple Cheap Requires no structured tool	Subjective Inaccurate Adherence estimates may affect/be affected by physician-patient relationship	No particular bias	De facto manner in which adherence is usually assessed Inaccurate both for predicting adherence and non-adherence [31-33] One study noted that physicians correctly rated their patients’ adherence 40% of the time [3]
Patient self-report	Simple Cheap Allows qualitative assessment of adherence	Subjective Inaccurate Accuracy can be affected by: poor patient recall, failure to recognize mistimed doses, dose missed over holidays/weekends as non-adherence, lack of patient candor	Overestimates adherence	Currently the most widely-used adherence measure More accurate for predicting non-adherence than high adherence [20] Encompasses a variety of techniques, including unstructured interviews, visual analog scales, and standardized questionnaires One study found that patients recalled only 41% of documented visits, while 28% recalled visits that never occurred [34] One study found that of patients who denied missing any protease inhibitor doses, 50% had undetectable levels [35]
Pill counts	Simple Cheap Objective	Accuracy can be affected by: throwing away remaining pills prior to seeing provider (pill dumping), inability to confirm who took pills, no information on timing of doses	Overestimates adherence	Frequently used in research alone or in combination with to patient self-report
Pharmacy refill records	Objective	Requires that patients bring in bottles Accuracy can be affected by: inability to confirm who took pills, inability to confirm timing of doses taken Requires capacity to maintain records and track patients over time	Overestimates adherence	Evidence has linked high refill rates with improved outcomes [36] Frequently used in research in addition to patient self-report
Drug level monitoring	Objective	Expensive Technically difficult (requires laboratory, testing capacity) Invasive (requires blood draws) Accuracy can be affected by: limited time frame of test effectiveness (3–4 days), inability to confirm timing of doses taken Requires baseline PK profile of population under study for accurate interpretation of results	Can overestimate or underestimate depending on: patient behavior immediately preceding test genetic variations in drug metabolism	One study found that patients with low ratios of observed to predicted concentrations of efavirenz were less likely to have UDVL [37]
Electronic drug monitoring	Objective Provides data on timing of doses taken Permits monitoring over long periods	Expensive Requires training, computer, operator, and specialized pill bottles Intrusive (patients may resent being monitored) Accuracy can be affected by: inability to confirm who took pills Incompatible with pill trays	Underestimates adherence (patients may take out multiple doses at a time for later use)	EDM more accurately predicts UDVL than self-report or pill count [19]

UDVL, Undetectable viral load; PK, Pharmacokinetic; EDM, electronic drug monitor.