RBCs in body fluids.

Q I have a question regarding cell counts on body fluids. Are there any fluids that do not require the red blood cell (RBC) count? How useful are they?

A RBC counts are useful in peritoneal fluid, pleural fluid, pericardial fluid, synovial fluid, and cerebrospinal fluid. First, no

RBCs should be present in body fluid under normal conditions. If numerous RBCs are present (except traumatic tap), it suggests a pathologic process. Malignancy, trauma, infarct, and hemorrhage are the major causes for bloody fluid. Second, correlating RBC with white blood cell (WBC) counts is helpful to determine if increased WBCs are due to peripheral blood contamination.

The value of RBC counts in fluids, however, is somewhat limited. No RBC reference value is available for determining hemorrhage in any fluid. A small number of RBCs (5,000/[micro]L to 10,000/[micro]L) will produce a blood-tinged fluid that may be found in transudates, exudates, and a small amount of bleeding. Higher RBC counts (>100,000/[micro]L) in pleural fluid are highly suggestive of malignancy, trauma, or pulmonary embolus/infarct. RBC counts are also used in peritoneal lavage fluids for assessing blunt and penetrating trauma as well as other conditions.

In summary, an accurate RBC count has a limited differential diagnostic value in body fluids. It should not, however, be entirely ignored either.

--Eric Nutt, MT (ASCP)

--Guang Fan, MD, PhD

Pathology Department

Oregon Health and Science University

Portland, OR

Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.

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