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D. J. Covert, R. Mancini, A. V. Mantravadi; The Relationship Between Diabetic Patients’ Subjective Estimates of Daily Capillary Blood Glucose and Objective Measures Including Hemoglobin A1C, Insulin Use, Type of Diabetes and Presence of Diabetic Retinopathy. Invest. Ophthalmol. Vis. Sci. 2007;48(13):168.
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This investigation was undertaken to describe the relationship between diabetic patients’ subjective estimates of daily capillary blood glucose (CBG) and objective measures of diabetic control and severity including most recent hemoglobin A1C (HgbA1C) and presence of diabetic retinopathy (DR).
This is an IRB approved observational study of patients with diabetes mellitus at a regional Veterans Affairs Medical Center. Patients who routinely checked their CBG at home were asked to estimate the average of recent measurements. Their last HgbA1C, presence of diabetic retinopathy, insulin use, and type of diabetes were noted.
This analysis included 231 consecutive patients with diabetes--seven with Type 1 and 224 with Type 2. The mean of patients’ estimates of CBG was 138 mg/dL and the mean of the most recent HgbA1C was 7.71. Diabetic retinopathy was present in 81 patients. Insulin therapy was present in 107 patients. A linear regression model between CBG and HgbA1C was statistically significant (p<0.0001) with coefficient of determination (r2) of 0.15. Using receiver-operator curve analysis, we determined a threshold value of CBG (130 mg/dL) above which it was likely that the HgbA1C was markedly elevated (>8.0), with 71% sensitivity and 61% specificity. Univariate analysis of CBG to predict DR using logistic regression revealed an odds ratio of 1.016 per one-unit increase in CBG (95% CI: 1.008-1.024). Multivariate analysis including CBG, HgbA1C, insulin use, and type of diabetes to predict DR revealed only insulin use as a statistically significant predictive factor with odds ratio 6.83 (95% CI: 3.42-13.6). Mean CBG was greater among those using insulin (+22.2 mg/dL, 95% CI: 13.3-31.1), Type 1 diabetics (+53.7 mg/dL, 95% CI: 27.5-79.9), and those with DR (+19.6 mg/dL, 95% CI: 10.2-29). Assuming a clinically established conversion of HgbA1C to average blood glucose in the preceding 10-12 weeks, we found that 87% of patients (200) underestimated their average glucose.
To the authors’ knowledge, this represents the first study to assess the relationship between subjective patient-derived estimates of CBG and objective measures of diabetes including HgbA1C. Patients’ estimates of CBG were positively correlated with HgbA1C, were somewhat predictive of diabetic retinopathy in univariate analysis, and were higher among those patients with Type 1 DM, those using insulin, and those with diabetic retinopathy. The vast majority of patients underestimated their average blood glucose.
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