As diagnostic tests rely on ever-tinier amounts of blood, some scientists are striking a note of caution. As it turns out, not all drops of blood are identical.

Bioengineers at Rice University recently found that different drops from single fingerpricks on multiple subjects varied substantially on results for basic health measures like hemoglobin, white blood cell counts and platelet counts.

Their study was published in The American Journal of Clinical Pathology.

To get results as accurate as those achieved by the traditional method — inserting a needle into an arm vein — the investigators had to average the results of six to nine drops, said Rebecca Richards-Kortum, the director of Rice 360°: Institute for Global Health Technologies, which did the research.

The investigators were careful not to squeeze or “milk” the subjects’ fingers, which has been known to invalidate results, said Meaghan Bond, the Rice bioengineering student who did the study with Dr. Richards-Kortum.