Accuracy of filter paper method for measuring glycated hemoglobin.

BACKGROUND AND OBJECTIVES Glycated hemoglobin (HbA1c) provides an accurate and reliable method to assess the glycemic control in patients with Diabetes. Its measurement is limited by the inconvenience of sample collection that requires venipuncture, sample handling and storage factors. The aim of this study was to assess the feasibility of using a dried capillary blood spot on a filter paper to estimate HbA1c, to check its stability at room temperature and to compare these values with the venous sample HbA1c by Turbidimetric Inhibition Immunoassay (TINA, Tina-quant HbA1c II). METHODS Venous blood samples of seventy eight patients with Type 1 or type 2 diabetes, were collected in EDTA containing vacutainers. Stability of HbA1c was studied in capillary blood samples blotted on to Whatman number 1 filter paper and stored at room temperature, for the first 20 patients enrolled in the study. After establishing the stability over a ten day period, HbA1c values obtained on the capillary blood spots were compared with those obtained from the venous blood samples of the remaining 58 patients. RESULTS Glycated hemoglobin is found to be stable in dried capillary blood spots on filter paper till the 10th day, stored at room temperature. It however, shows an inherent variability of +/- 15%, which falls within the permissible variability (18%) of the quality control material. Seventy nine percent of the capillary HbA1c values were found to fall within this range. With linear regression, we derived the relationship between filter paper and venous HbA1c values. The regression equation was as follows: Cap.HbA1c = 0.95 (Ven.HbA1c) + 1.4. The filter paper results were highly correlated with the venous sample values (r = 0.889, p < 0.01). CONCLUSION Measurement of glycated hemoglobin in dried blood spots on filter paper gives reliable and reproducible results. In our study, the mean capillary sample HbA1c value was 12% higher compared to the venous sample HbA1c values. Therefore a higher normal range may have to be used for interpreting the dried blood spot capillary blood HbA1c values.