Valid assessment of immunosuppressive therapy non-adherence (NAH) is vital: NAH is associated with negative transplantation outcomes. We studied the diagnostic accuracy of assay, patient self-reports and clinicians' collateral reports and composite adherence scores using electronic monitoring (EM) as a reference standard. This cross-sectional study included a convenience sample of 249 adult kidney transplant recipients (Ktx) (female: 43.4%; mean age 53.6 [SD: 12.7], median 7 years [IQR: 9 years] post-Ktx). NAH was assessed using EM over 3 months (i.e. reference standard), assays of cyclosporine, tacrolimus, mycophenolat-mofetil, patients' self-reports and clinicians' collateral reports. The constructed composite adherence score included assay, self-reports and collateral reports. NAH's prevalence across the measurement methods was EM: 17.3%; assay: 33% (cyclosporine: 25.8%; tacrolimus: 35.1%; mycophenolat-mofetil: 40.2%); self-report: 12.4%; collateral reports: 24.9% and composite adherence score: 38.9%, respectively. The composite adherence score and collateral reports showed the highest and lowest sensitivities to NAH (72.1% and 15.8%, respectively). Specificity was highest for collateral reports of at least three clinicians (93.1%). Likelihood ratio of a positive test was 2.74 for composite adherence score. No measures showed high sensitivity alongside high specificity. Combining measures increased diagnostic accuracy, indicating the relevance of combined measures for clinical and research purposes.