A new study suggests health care professionals should be cautious when using measurements from a single drop of fingerprick blood. The researchers say to achieve consistent test results, as many as six to nine drops of blood should be combined.

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The researchers showed that results varied significantly from drop to drop in fingerprick blood tests.

The study, published in the American Journal of Clinical Pathology and believed to be the first of its kind, is the work of two bioengineers at Rice University in Houston, TX - Rebecca Richards-Kortum, a professor of bioengineering and of electrical and computer engineering, and one of her students, Meaghan Bond.

The study shows that measurements from a single drop of blood are highly variable. The finding is important, say the authors, because devices that use only one drop of blood to give test results are becoming increasingly available.

The study came about because of work Bond was involved in helping to develop new, low-cost tests for anemia, platelet and white blood cell counts.

The most accurate way to test for these properties is to draw blood from a vein and send it to a lab.

But sometimes, such as in low-resource settings, results are needed more rapidly, or the health professional is not trained to draw vein-blood, or there is no lab. In such settings, there is a higher reliance on fingerprick tests.

Bond noticed there was wide variation in some of the benchmark tests they were performing using hospital-grade blood analyzers to gauge the accuracy of the new technologies for fingerprick tests.

But the authors could not determine whether this problem was a result of the experiments themselves or actual variations in the properties of the blood droplets - such as amount of hemoglobin, platelets and white blood cells.

Results varied significantly from drop to drop

For their study, the researchers tested six droplets of blood (20 microliter droplets) drawn in succession from each of 11 donors.

Drawn in succession means that the droplets came from the same fingerprick. The researchers note they also followed best practice to ensure accurate results. For example, they wiped away the first droplet to remove traces of disinfectant, and they did not squeeze or "milk" the finger.

The researchers also ran another test with a separate set of donors to check whether the minimum droplet size made a difference. And they checked all the results against blood taken from donors' veins.

The results showed that hemoglobin content, platelet count and white blood cell count varied significantly from drop to drop.

Bond says they were surprised by some of the results. For example, "in some donors, the hemoglobin concentration changed by more than two grams per deciliter in the span of two successive drops of blood," she adds.

She notes that it is important for health care professionals to understand that different protocols can affect the results of fingerprick tests, and also how the results of the same test can vary in the same patient.

While acknowledging the importance and usefulness of fingerprick blood tests - particularly in low-resource settings - Bond urges that:

"Our results show that people need to take care to administer fingerprick tests in a way that produces accurate results because accuracy in these tests is increasingly important for diagnosing conditions like anemia, infections and sickle-cell anemia, malaria, HIV and other diseases."

The authors found that averaging the results of six to nine successive droplet tests produced results on a par with the vein blood tests.

They discuss their study and its implications in the following video:

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