Each 10-microlitre droplet was tested for hemoglobin concentration with a popular point-of-care blood analyser used in many clinics and blood centres.
Finger prick blood tests that use only a single drop of blood may yield inconsistent results, a first-of-its-kind study has found.
The findings suggest that health care professionals must take care to avoid skewed results as they design new protocols and technologies that rely on finger prick blood.
"We began looking at this after we got some surprising results from our controls in an earlier study," said lead investigator Rebecca Richards-Kortum from Rice University in US.
The researchers noticed that there was wide variation in some of the benchmark tests that they were performing on hospital-grade blood analysers.
The benchmark controls are used to gauge the accuracy of test results from a new technology under study, so variation among the control data was a sign that something was amiss.
The researchers designed a simple protocol to test whether there was actual variation, and if so, how much. They drew six successive 20-microlitre droplets of blood from 11 donors.
As an additional test to determine whether minimum droplet size might also affect the results, they drew 10 successive 10-microlitre droplets from seven additional donors.
Each 20-microlitre droplet was analysed with a hospital-grade blood analyser for hemoglobin concentration, total white blood cells (WBC) count, platelet count and three-part WBC differential, a test that measures the ratio of different types of white blood cells, including lymphocytes and granulocytes.
Each 10-microlitre droplet was tested for hemoglobin concentration with a popular point-of-care blood analyser used in many clinics and blood centres.
Researchers found that hemoglobin content, platelet count and WBC count each varied significantly from drop to drop.
"Some of the differences were surprising. For example, in some donors, the hemoglobin concentration changed by more than two grams per decilitre in the span of two successive drops of blood," said Meaghan Bond, a student at the Rice University.
Researchers found that averaging the results of the droplet tests could produce results that were on par with venous blood tests, but tests on six to nine drops blood were needed to achieve consistent results.
"Our results show that people need to take care to administer finger prick tests in a way that produces accurate results because accuracy in these tests is increasingly important for diagnosing conditions like anaemia, infections and sickle-cell anaemia, malaria, HIV and other diseases," Bond said.
The study was published in the American Journal of Clinical Pathology.