Psychometric analysis of the 43 item version of the DNT shows that it has good reliability and validity in testing numeracy skills in patients with diabetes. Of the six a priori construct hypotheses, five demonstrated significant correlations in the expected directions. The DNT was correlated with diabetes knowledge, education, socioeconomic status, literacy and general numeracy. Insulin use was not correlated with the DNT scores as initially predicted. This is likely due to the fact that many patients on insulin may be placed on long-acting insulin in which one or two doses may be administered with no adjustment for blood glucose or carbohydrate intake. When complexity of the insulin regimen was taken into account, more complex regimens including adjustment for carbohydrate intake and blood glucose level were significantly associated with the DNT.
The shortened version, the DNT15, also showed good reliability and construct validity. The DNT15 was designed to retain the items that discriminated diabetes related numeracy skills while also keeping the items that would be most useful to a diabetes educator or clinician. The five diabetes self-care areas are retained in the DNT15, including three items on nutrition, one item about exercise, three items regarding blood glucose monitoring, one item on oral medications and seven items about insulin administration. A diabetes educator or clinician may use the DNT15 to help target education or guide therapy.
Patients with low literacy also showed low numeracy skills on both the WRAT and the DNT. However, there were several patients with literacy skills above the ninth grade level who had low numeracy skills. Although our population was, on average, highly educated, the mean score on the DNT was only 61% correct. This suggests that numeracy should be evaluated separately from literacy. Patients with low literacy need special instructions and interventions, and patients with inadequate numeracy skills may also require modified counseling and education to improve health outcomes.
This study has several limitations. The DNT was initially developed without input from patients to determine what numeracy skills patients viewed as important. We used a convenience sample of patients recruited from academic and VA clinics, and it is possible that patients who did not participate in the study may have had different literacy and numeracy skills than those that participated. The DNT has been tested in English speaking participants only. The DNT was, in part, validated against a commonly used literacy assessment tool (the REALM), but most patients in the study scored at the highest level that the REALM assesses. A more refined assessment of reading ability at the higher levels may have been more useful.
The DNT and DNT-15 are primarily research tools for understanding the role of diabetes specific numeracy in the management of diabetes. The clinical utility of these scales will be the subject of future research. The treatment of diabetes requires the application of many literacy and numeracy skills. As the disease progresses, the complexity of the regimen may also progress. Patients need ongoing education to appropriately treat their diabetes. However, little is known about the benefits of targeting education to a patient's level of diabetes specific numeracy to improve health outcomes. Other studies have identified the role of literacy and low-literacy techniques in the improvement of health outcomes in diabetes and congestive heart failure [19, 20]. Patients with low numeracy may benefit from similar interventions that address numeracy, particularly in the setting of diabetes management. The DNT and the DNT15 can provide measurement of diabetes specific numeracy and provide more information about the role of disease specific numeracy in future studies. More studies are needed to further understand the role of numeracy tailored interventions in the management of diabetes.
However, there are also clinical implications that can be learned from this study. We learned that the framing of instructions was very important in predicting patient performance. For example, study participants had a difficult time with the multi-step math required to calculate a correction dosage of insulin when instructions were presented as a sequence of sentences. This item was included to mirror clinical practice regarding how patients are currently instructed to take their insulin. Patients clearly had a much easier time calculating the insulin dosage when the material was presented in an easy to read table. This example provides an important lesson for health care providers and educators in effective communication styles for all clinical care recommendations.