The relationship between blood glucose levels in oral glucose tolerance test and HbA1c

Aims: The importance of HbA1c in the diagnosis of prediabetes and diabetes; investigation the effectiveness of HbA1c in screening and diagnosis, determining the sensitivity and specifity of HbA1c in patients whom conducted oral glucose tolerance test.Methods: The people whom conducted oral glucose tolerance test between June 2010 and September 2012 at Dicle University Faculty of Medicine, Department of Endocrinology were studied. The patients with gestational diabetes excluded from the study. The study group consists of 560 people [363 persons (64.5%) female and 197 people (35.1%) were male] between the ages 15 and 84. In this group from 232 people; 0,30,60,90,120. minutes and from others 0 and 120. minutes blood glucose level was measured. Concurrently with the blood glucose test HbA1c levels were studied by the HPLC method and the results were compared.Statistic Analyses: Chi-square, Pearson correlation, Student-t test, One way ANOVA, Post-hoc tests (Bonferroni correction), ROC curve were used. Chi-square test was used to compare groups, p <0.05 was considered statistically significant. The ROC curve was used to show the accuracy rate of test in distinguishing patient and non-patient individuals.Results: In our study, 129 people (23%) had been diagnosed with diabetes. In patients with diabetes; mean age 50.7 ± 11.4, mean HbA1c 6.72% ± 1.06, mean AUC were 474.3 ± 103.5. OGTT 90. minute with HbA1c correlation r: 0,464 p: 0,001 and AUC correlation were r: 0,971 p: 0.001 respectively. For the diagnosis of diabetes HbA1c 6.5%; 55.0% sensitivity and 80.9% specificity, PPV:46%, NPV:85% (p:0.001) was found. HbA1c 5.7%; 88.3% sensitivity and 36.4% specificity, while 7.6% the specificity was 100% by HbA1c. For the diagnosis of prediabetes with HbA1c 5.7%; 70.0% sensitivity, 42.2% specifity, PPV:52%, NPV:61% respectively (p:0,001). However, the optimal cutoff value in the diagnosis of prediabetes for HbA1c level of 5.9%; 58.5% sensitivity and 61.7% specifity was found.Conclusions: HbA1c level 6.5% in the diagnosis of diabetes and HbA1c level 5.7% in the diagnosis of prediabetes is not a suitable parameter due to the low sensitivity and specificity in our society. Although the cost of HbA1c is higher than fasting blood glucose, HbA1c can be useful in the management of diabetes and help to prevent the complications of diabetes. Fasting blood glucose is only used in diagnosis and screening of diabetes, however using fasting blood glucose and HbA1c measurement together can reduce the sensitivity and cause more expenditure; so it is not appropriate to use fasting blood glucose and HbA1c measurament together. The sensitivity and specificity for optimal cut-off value of HbA1c 6.1% in the diagnosis of diabetes was not found satisfactory. Measurement of HbA1c in the diagnosis of diabetes does not seem to provide additional benefit but it can be used in some occasions which the oral glucose tolerance test can be harmful (eg Heart diseases, CVO) ,also HbA1c value 7.6% can be used in diagnosis of diabetes for intensive care patients. OGTT 90. minute level is best correlated with AUC and HbA1c levels. Using OGTT 90. minute level as a criterion for the diagnosis of diabetes may provide additional benefits by reducing duration needed for diagnosis.Keywords: DM, oral glucose tolerance test, HbA1c, prediabetes, impaired fasting glucose, impaired glucose tolerance.