The results of the selection and data extraction of the studies are presented in the PRISMA flow diagram (Fig. 1). The searches done in July 2020 resulted in a total of 41,886 records published between 1973 and June of 2020 were considered potentially eligible and retrieved from eight databases. A total of 14,826 duplicates were removed. The titles and abstracts of 27,060 records were peer-reviewed by three pairs of peer reviewers, who evaluated 9,020 records each. A total of 336 records were identified for full-text assessment and 84 records were included. Eight additional relevant records were added after manually searching the lists of references from the included studies.

PRISMA flow diagram. Note: ARMS = Adherence to Refills and Medication Scale; AS = Adherence Scale; DMAS-7 = 7-item Diabetes Medication Adherence Scale; GMAS = General Medication Adherence Scale; IADMAS = Iraqi Anti-Diabetic Medication Adherence Scale; LMAS-14 = Fourteen-item Lebanese Medication Adherence Scale; MALMAS = Malaysian Medication Adherence Scale; MAQ = Medication Adherence Questionnaire; MARS-5 = 5-item Medication Adherence Report Scale; Mascard = Medication Adherence Scale in Cardiovascular disorders; MEDS = Medication Adherence Estimation and Differentiation Scale; MGT = Morisky-Green test; MMAS-5 = 5-item adapted Morisky Medication Adherence Scale; MMAS-8 = 8-item Morisky Medication Adherence Scale; MNPS = Medication Non-persistence Scale; MTA = Measurement of Treatment Adherence; MTA-Insulin = Measurement of Treatment Adherence—Insulin; MTA-OA = Measurement of Treatment Adherence—Oral Antidiabetics; PROM = Patient-reported outcome measures; ProMAS = Probabilistic Medication Adherence Scale; SMAQ = Simplified Medication Adherence Questionnaire

The update done in May 2023 resulted in 11,538 records published between 2020 and 2023 where 4,370 duplicates were removed. Out of 52 records assessed for full-text, 18 records were included resulting in a total of 110 records and 27 PROMs included in the systematic review (Fig. 1).

The included studies were published between 1986 and 2023, and most of them were conducted in the United States in the English language (n = 19). The sample size of included studies ranged from 30 to 6,261 participants. The percentage of females ranged from 17.0% to 79.3% and the mean age ranged from 43.1 to 81.9 years. About half of the studies were conducted in hospital settings (n = 54) and observed an average disease duration of 9.8 years (n = 26) and an average of 4.5 medications in use (n = 29).

Most of the 27 PROMs included in the review are one-dimensional and composed of items with a Likert-type scale response. A total of 39 studies [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66] conducted with patients who only had TD2M, another 47 studies [67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113] with patients who only had CVD, and the remaining 24 studies with patients having both T2DM and/or CVD [114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137]. The most prevalent original language of the 27 PROMs included in this review was English (n = 15). In addition to the original versions, translated versions of the PROMs were also included in the review. Of 27 PROMs included, 10 had been translated into at least another language. Regarding the application characteristics, for the majority of PROMs (n = 20) it was not clear what the recall period was (Table 3). It was not possible to describe the time to complete the PROMs because the majority of the studies did not present this information.

A total of 38 studies reported a response rate for PROMs that ranged from 21.1% to 100.0%. Out of the 110 records, only 63.6% of the studies presented information about conflict of interests, and 68.2% informed if the research had any source of funding (Additional file 3).

The two reviewers (HCO and RCMR) are authors of one of the included studies [120]. The analysis of this article was done by a third reviewer (NMCA).

The summary of findings of measurement properties of the PROMs is presented in Table 4. The summary of the assessment of risk of bias can be found in the Additional files 4 and 5. The results based on each measurement property of the PROMs are presented below.

The content validity resulted in overall ratings per PROM for relevance, comprehensiveness and comprehensibility, and overall content validity of the PROM. The indeterminate ratings for development or content validity studies were ignored in the overall rating assignment (n = 30). All the PROM development studies were classified as having inadequate methodological quality except the studies that developed the PROMs ProMAS [130]. and Mascard [