Clinical Laboratory professionals know that point of care test results performed on a fingerstick sample of blood are often less accurate and precise than results performed on a central laboratory analyzer using a venous sample. Improvements in point of care technology have greatly improved the analytical performance of point of care devices. A recent study published in the American Journal of Clinical Pathology convincingly demonstrated that much of the variability in point of care testing occurs during the preanalytical phase of collecting a fingerstick sample of blood.

Rebecca Richards-Kortum, a professor of bioengineering and of electrical and computer engineering at Rice University in Houston, Tx and one of her students, Meaghan Bond, measured the variability of CBC results obtained on a hematology analyzer (Beckman Coulter Ac-T diff2) from six consecutive blood drops collected during a single fingerstick . They repeated this experiment on a total of 11 different individuals.

Compared to tests on venous blood, the coefficient of variation for successive drops of fingerstick blood exceeded venous blood by 3.4 times for hemoglobin, 5.7 times for WBC count, 3.0 times for lymphocyte count, 7.7 times for granulocyte count, and 4.0 times for platelet count. In some individuals, hemoglobin concentration differed by more than two g/dL between two successive drops of blood.

The authors found that averaging the results of six to nine successive droplets produced results on par with venous blood samples. High rates of variability from one drop of blood to another raise questions about the reliability of point-of-care results performed on capillary blood. For example, how many RBC transfusions have been ordered based on an erroneous fingerstick hemoglobin result?

Bond MM and Richards-Kartum RR, Drop-to-Drop Variation in the Cellular Components of Fingerprick Blood. Am J Clin Pathol 2015;144:885-94.


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