Adequate health literacy is crucial for patients to make optimal choices for their health and medications management. Additionally, successful health communication presupposes certain levels of competence of both the healthcare professional and the patients and is adapted to the HL of the individuals (5).

“HL” as a recognized term came into use around 1974, but only became a [Medical Subject Headings (MeSH)] term in MEDLINE in 2006. A systematic review of Sørensen et al. (17) discovered 17 definitions and 12 conceptual models of HL. Based on all the offered, one overall definition was obtained, which reads: “HL is linked to literacy and entails people's knowledge, motivation and competencies to access, understand, appraise, and apply health information in order to make judgments and take decisions in everyday life concerning healthcare, disease prevention and health promotion to maintain or improve quality of life during the life course.” There is currently no consensus on defining HL, which means that different approaches to this term are used in various research environments (17–20). An additional concept of understanding becomes a problem when it comes to assessing and measuring HL and then comparing these results between different studies (16). The presence of different definitions is probably due to the fact that the concept has been developed in different parts of the world, with varying abilities and skills considered necessary to deal with health information in each specific context (21).

In the twentieth century, reading and writing were sufficient to use information obtained from health professionals. However, with increasing expectations of active participation in health decisions, increasing responsibility for one's health, and digital development in health information, additional skills are needed to handle health information. First, the need for reading has increased, and the skills to apply and critically evaluate health information from various sources are essential. HL combines a set of skills or abilities, while on the other hand it depends on the requirements to which the individual is exposed. The impact of technological development has also increased, which will affect the definition and understanding of HL in the future (22).

Patients with chronic diseases need to continuously perform tasks associated with self-care and self-management of their medications. When taking medicines, they constantly need abilities related to various domains of HL, so HL brings together many concepts that are associated with patient's pharmacotherapy. Whether they rely on information in printed materials or verbal instruction patients with chronic conditions need to have adequate HL related to medications as critical for managing their conditions. Due to the complexity of the various procedures required for the adequate use of medications, the concept of PTHL was introduced.

King and colleagues, in consultation with the academy and pharmacists, formulated PTHL as: “An individual's capacity to obtain, evaluate, calculate, and comprehend basic information about pharmacotherapy and pharmacy related services necessary to make appropriate medication-related decisions, regardless of the mode of content delivery (e.g., written, oral, visual images and symbols)” (23). This definition was updated by adding to reduce thereby the risk of poor pharmacotherapy outcomes (24).

Diabetes is one of the most common chronic non-communicable diseases and is a major public health problem. In 2019, the International Diabetes Federation estimated that 463 million adults worldwide have diabetes and that this number is expected to increase to 700 million by 2,045. The cause of this disease is multifactorial, but it is associated with unhealthy lifestyles such as physical inactivity and poor diet. It is assumed that between 30 and 80% of people with type 2 diabetes are still undiagnosed. Complications such as diabetic nephropathy and neuropathy may occur at a later diagnosis of disease (25). Despite advances in therapy and the availability of clinical practice guides, only 30% of patients manage to achieve the target values of glycemia, cholesterol and blood pressure. The fact is that patients perform 95% of diabetes care on their own (26).

Type 2 diabetes is more common than type 1, so 90% of all diabetes is type 2 diabetes. It most often occurs in middle age and in the elderly. It is closely related to lifestyle and health habits, with being overweight and obese being risk factors. Hereditary factors can also influence the risk of developing this disease. Therapy includes weight loss, diet and therapy with drugs that lower blood glucose levels. Affected people are advised to give up cigarettes and reduce alcohol intake to prevent the appearance of cardiovascular diseases. Living with this disease requires changes in health behavior, self-control, and a lot of care (27–29). Since living with type 2 diabetes requires a lot from people with the disease, these persons must be informed about therapy, diet and other health behaviours, which require adequate HL and PTHL. Several international studies have shown that reading and understanding the guidelines for modern diabetes medications, applying appropriate dietary restrictions, and gaining insight into the physiological processes involved in the disease can be a major challenge for an individual (30, 31). The performing of diabetes self-management tasks frequently involves abilities, such as taking medications at the right time, interpreting blood glucose levels and calculating insulin doses.

A recent review of HL and health outcomes in patients with type 2 diabetes concludes that there is strong evidence to suggest a positive correlation between HL and diabetes knowledge (32). It is also considered that there is sufficient evidence to support a link between HL and self-care (33). On the other hand, the evidence of a link between HL and clinical indicators was inconsistent (34).

Some primary studies that looked at the level of HL in patients with type 2 diabetes found that a small number of these patients had adequate levels of HL (35–39).

Patients with diabetes and limited HL and PTHL often cannot read drug medication labels correctly, may misuse their medication, do not understand the meaning of consent forms, and generally have difficulty understanding printed care instructions and reading health advice and warnings (40–43). For this reason, is very important to assess their PTHL and in case of needs perform adequate training in order to improve control of their disease and pharmacotherapy management.

These patients also have poorer communication with doctors and participate less in making health decisions (4). Patients who are diagnosed have to make health decisions daily and must also perform complex self-care activities to keep the disease under control. Interventions in upgrading HL education and intensive diabetes-related education have shown good results in patients with limited HL to improve diabetes outcomes (44, 45). People with type 2 diabetes should undergo diabetes education programs at the time of diagnosis and then once a year. This education aims to enable individuals to participate in informed decision-making and disease control, all with the aim of better outcomes in treatment of this disease, improvement of glycemic control, prevention of complications and comorbidities, and improvement of quality of life (46). Education for diabetics should be evidence-based, have specific goals, and be tailored to the needs of individuals. However, the effectiveness of this education depends on individuals, i.e., characteristics such as age, gender, ethnicity, level of HL, ability to take care of themselves, all of which should be taken into account when planning and implementing this type of education (27, 47). In this way, they will be able to understand and use the information they receive to maintain health and control diabetes in everyday life (47).

In the past 25 years, numerous instruments have been developed to measure HL and PTHL in various contexts (14, 24, 48). These instruments significantly differ in structure, measurement, range, and psychometric properties. The diversity of instruments has led to inconsistencies in measurement with the complexity of interpreting the results and choosing suitable instruments for new studies. Several studies have examined the variation through the range of the most used HL instruments (49, 50). Such variations can come from the fact that the instruments measure different conceptual dimensions of HL. However, it may be difficult for health professionals or researchers to choose the best instrument when they are unfamiliar with measurement properties. Another very important consideration in selecting a HL instrument is its mode of administration. In a subjective instrument, individuals self-report their perceived levels of literacy skills, such as using Likert scales. In contrast, an objective instrument is the interviewer-administrated instrument and assesses the ability to process information by asking respondents to answer specific questions, such as about the time to take the next medication. A subjective instrument requires less cognitive effort in responding to questions, whereas an objective instrument assesses health numeracy more accurately. A self-administrated instrument can be more practical in a very busy clinical settings, than interviewer-administered instrument. Sometimes, the interviewer-administrated instrument may result in discomfort or embarrassment for patients who have a low HL.

Although the instruments were used in several populations, due to the complexity of the tasks and skills that people with type 2 diabetes require, their usefulness and applicability for this population remain challenging. With the growing interest in this construct, there has been an increasing demand for valid and reliable instruments for estimating HL and PTHL.

A systematic review of measurement properties has been designed for providing a comprehensive overview of the available instruments and identifying the best currently available instrument for general population (51). In the previous reviews of HL instruments methodological limitations were identified, such as being descriptive rather than systematic reviews, or lacking quality assessment or data synthesis (14, 52, 53). To address these limitations, a scoping review was conducted to systematically collect the literacy instruments used in people with type 2 diabetes and meet needs for understanding the characteristics, scope of measurements, and their applicability in this population.

This study aims to analyze instruments used in patients with type 2 diabetes mellites for measuring HL and PTHL, in relation to their characteristics (measurement scope, structure, domains, method of scoring), validation, strengths, limitations and accordingly to propose applicability of these instruments in clinical and research settings. This work can be useful as an inventory for researchers and practitioners who are seeking to identify validated measurement instruments in patients with type 2 diabetes mellitus and other chronic diseases that are fitting the best for their research and practice.

Authors built a search strategy by using the PICOS questionnaire. During November and the first 2 weeks of December 2020, a systematic search of the Scopus and PubMed databases was performed in search of peer-reviewed literature of patients with type 2 diabetes mellitus. The protocol of this systematic review (including the article identification strategy and data collection form, etc.) mainly referred to the Cochrane Handbook for Systematic Reviews of Interventions (54) and the reporting of this systematic review was based on the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement (55).

The PRISMA flow chart in Figure 1 summarises the results of the search process. 1. A search obtained 5,874 potentially relevant studies, while 8 were found by reference review. A cursory review of the content of the papers left 356 papers for further evaluation. Further exclusion (after reading the abstracts and methodology) was based on the principle of excluding papers that mentioned the instrument for assessing HL and PTHL, but without analyzing its structure, and excluding papers that are duplicates (same author, same instrument) left 111 articles for full reading, of which the final number of included studies was 24.

The comparison method was used to compare the instrument in terms of their structure (number and type of questions), the way of reporting, version, purpose, place where they were developed, target population, the person who developed it, year of publication, scoring, heath literacy domains, time and way for administration, measurement scope, validation, strengths, and limitations.

A thorough analysis has been presented in the Tables 3–5. Tables 3, 4 present the instruments used in the studies to assess HL and PHTL, their basic characteristics: domains, methods of assignment, structure and method of scoring. Table 5 shows the psychometric characteristics of the instruments, strengths and limitations.

The identified instruments used for assessing the level of HL and PTHL in extracted works are the following: Korean Functional Test HL (1 study), Newest Vital Sign, NVS (1 study), Functional, Communicative and Critical Health Literacy scale, FCCHL (1 study), Health Literacy Survey European Questionnaire 47, HLS-EU-47 (1 study), Test of Functional Health Literacy in Adults, TOFLHA (1 study), Test of Functional Health Literacy in Adults–Short Form, S-TOFHLA (8 studies), Rapid Estimate of Adult Literacy in Medicine–Revised, REALM-R (1 study), 3 brief screening questions, 3-brief SQ (6 studies), Rapid Estimate of Adult Literacy in Medicine, REALM (3 studies), Health Literacy Questionnaire, HLQ (1 study) and Diabetes Numeracy Test 15, DNT-15 (1 study) (37, 58–80). There are considerable differences found in their structure, number of items, administration time, available languages, type of administration, scoring system, measurement scope and properties, implicating their use in different settings.

The most used measures of HL in patients with type 2 diabetes mellitus were S-TOFHLA, then 3-brief SQ, REALM and HLS-EU-47. The S-TOFHLA, TOFHLA and REALM have been validated in different populations and are used in validation studies for 3-brief SQ and NVS. They are considered as a gold standard in validation studies. However, 3-brief SQ and NVS have a broader measurement scope and better properties, which put them in a better position for use in future validation studies. S-TOFHLA, REALM and HLS-EU-47 are not practical in busy clinical settings and REALM requires researcher participation. While the most used instruments, S-TOFHLA and REALM, measure only the functional domain of HL, the others 3-brief SQ, and HLQ address functional and critical HL, and DNT-15, KHLS and TOFHLA functional HL and numeracy. The only one for examination of all three levels of HL individually, their mutual correlation and different effects on patient is FCCHL.

NVS - Good reliability and convergent validity with well-validated and commonly used measures of HL such as the TOFHLA. Strengths are related to its suitability for rapid assessment of low HL. Test format might intimidate respondents.

FCCHL - Strong positive evidence for its content and structural validity and moderate positive evidence for internal consistency. This scale includes three levels of HL, each of which might have different effects on patient outcomes. It is proved to be easy to administer in a clinical setting. The scale is not validated in English.

Three-brief SQ - Positive evidence for the criterion validity of the 3-SQ with the S-TOFHLA (36 items) and limited negative evidence for its hypothesis testing validity and internal consistency. Instrument is validated in several diverse sample populations. It is quick, easy, and inexpensive for administration. Limitation is related to self-assessment and potential for self-report bias.

HLS-EU-47 - High levels of internal consistency reliability. It is available in many languages, length of assessment increases response burden.

S-TOFHLA - Demonstrated evidence for the internal consistency due to there being no evidence of structural validity. It has been validated in several diverse populations. Lack of this instrument is that it may not capture an individuals' HL in the dimension of numeracy.

REALM - Found to have a high test-retest reliability of 0.99. It is assessed by healthcare professionals and was found to have good face validity; however, it lacks in construct validity. It is quick and easy for administration, limited to the ability to pronounce words without being able to measure the patient's ability to understand the instructions on labelling of prescribed drug.

HLQ - Positive moderate evidence for its content validity and internal consistency and unknown evidence for structural validity. Strength is that measures multiple domains of HL, but due to self-assessment has a potential for self-report bias.

DNT - Moderate evidence for its content validity and internal consistency and limited positive evidence for structural validity. This is test numeracy that is associated with diabetes management. Limitation is that can be difficult or require high numeracy skills (49, 78, 81–89).

Preliminary data in relation to HL and PTHL were extracted from the studies included in the qualitative synthesis and summarized in Table 6 (for HL) and Table 7 (for PTHL).

In the research period most of the studies were published in the period from 2006 to 2021 (37, 58–80). The largest number of studies was conducted in the United States (11 studies) and Canada (2 studies). In contrast, in South Korea, Japan, Switzerland, Taiwan, Spain, Norway, Brazil, Denmark, Iraq, Netherlands and Hungary, one study was performed. Cross-sectional studies make up the majority (20 studies), while three are longitudinal and one is a cohort study. The sample size ranged from at least 50 to 46,354 subjects and are adults over the age of (37, 58–80).

In six studies, limited HL was observed in <50% of subjects, and in five studies, more than 50% of subjects had limited HL. HL levels have been linked to diabetes knowledge and treatment efficacy, while HbA1C concentrations, Internet use, glycemic control, and health consequences have not been linked to HL levels (37, 58–80). One study found an increased prevalence of people with limited HL who were diagnosed with depression (67), while another study found that people with limited HL were more likely to eat unhealthily and had reduced physical activity compared to people with high HL (62).

As instruments for measuring HL and PTHL continue to be published, authors advise clinicians, health professionals and researchers to evaluate available HL and PTHL measurements for a conceptual and practical match with the goals of their work.

When choosing a practical match, style of administration, purpose for measurement, their basic characteristics: domains, methods of assignment, structure, method of scoring, validation, strengths and limitations should be considered. It is important to align with the topic or task under consideration and choose the one that has been validated in a similar target population in order to have an accurate measure of the domain being assessed. Predictive qualities and appropriateness for assessment of changes in HL and outcomes over time have to be taken under consideration.

FCCHL was evaluated as the most appropriate instrument to apply to people with diabetes since a diabetes-specific type of instrument and the contents of its items may be more sensitive in a diabetes clinical setting targeted at diabetes, it is a model-based and comprehensive measure which covers all 3 levels of HL and the evidence for the measurement properties are better than those for the other instruments. However, his structural validity needs to be further established, and therefore adding DNT-15 questionnaire can be one of the options for considering application of FCCHL in this population (90).

Based on the previous considerations FCCHL and 3-brief SQ have the broadest measurement scopes. They are quick, easy, and inexpensive for administration. Three-level HL can be considered as the most useful and comprehensive instrument to screen for inadequate HL. The limitation is that the English version is not validated. Three-brief SQ has many advantages in comparison to other instruments, including that it is less likely to cause anxiety and shame. This instrument can be considered the best for measuring functional HL in patients with diabetes mellitus type 2 and other chronic diseases.

This review was subject to some limitations. The use of non-interventional studies, the heterogeneity of studies in terms of samples represent important limitations for this scoping review. Only works written in English have been considered. Assessments of the instruments' dimensions, strengths, and limitations were made on the basis of our own experience and judgment; and as such, this was a subjective review. In order to minimize the effect of the issue of subjectivity, each measure was analyzed by multiple authors, and any discrepancy was addressed by all the authors and resolved through fruitful discussion.