First or Second Drop of Blood in Capillary Glucose Monitoring: Findings from a Quantitative Study

For clinical nurses, especially those working in emergency departments, it is crucial to measure blood glucose (BG) in an accurate, timely, and safe manner. Many differences in practice exist with regard to use of the first or second drop of blood for testing, and no consistent guidelines are available for capillary BG testing at home or in ED settings. The purpose of this study is to evaluate the BG differences between the first and second drop of capillary blood collected from the same site in patients with type 1 diabetes.

A consecutive sample of 195 persons with type 1 diabetes who had washed their hands and were not suspected of having hypoglycemia were included in the study. Descriptive and inferential statistical analysis for non-normal distributed variables was performed.

A strong correlation emerged between the BG reported in the first and the second drops (Spearman's rho test [rs] 0.979, P < .001; Pearson r 0.978, P < .001). The average BG values obtained from the first and second drops were 184.30 mg/dL (median, 166) and 187.6 mg/dL (median, 172), respectively, and thus the second drop showed higher glucose values compared with the first drop. However, BG values of the second drop were not higher in all occasions: whereas some evaluations reported higher BG values in the second drop capillary sample (n = 123), others reported higher values in the first drop (n = 65), and still others reported identical measurements in the first and second drops (n = 7). Five outliers were present with a BG difference from -39 to -53 mg/dL in the first drop compared with the second drop, and 3 outliers were present with a BG difference from +46 to +57 mg/dL in the first drop compared with the second drop. However, the differences that emerged were not affected by glucose concentration (P = .221).

Using the first drop of blood in a patient with clean hands allows emergency nurses to perform the test more quickly, resulting in immediate information. Findings indicate that the first drop of blood is adequate for clinical decision making, but the clinician should use judgment if using protocols in which small values (eg, 6 mg/dL) are important, because the first drop is more likely to have a slightly lower value.