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Editor: Frederick L. Kiechle, MD, PhD

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Q. A number of questions have arisen in our laboratory as to how automated body fluid cell counts should be reported. We use a Sysmex XE-5000, which enumerates WBC-BF for white cells, RBC-BF for red cells, and TC-BF# for total nucleated cells. We report only the WBC-BF and RBC-BF. Using the WBC-BF, we manually perform a 100-cell differential, including WBCs and nonhematological cells. Should we report out the TC-BF# instead of the WBC-BF since we are including all nucleated cells in our differential? Should we report both WBC-BF and TC-BF#? Would TC-BF#, as opposed to WBC-BF, have significance for our clinicians? Should the WBC-BF be used while performing a differential that includes only WBCs and a comment added about the presence of nonhematological cells?
A. January 2020—Most critically, the cell count and differential count should be evaluating the same cell types. If a total nucleated cell count is given, then the differential should include other non-WBCs so as to avoid overestimating the absolute count for each WBC type. Conversely, if a WBC count is given, then the differential should include just WBCs so as to avoid underestimating absolute counts of different leukocytes, most critically neutrophils.

Whether or not non-WBCs need to be counted is less clear. Body fluid cell counts traditionally have been performed on chamber counters, which do not reliably differentiate white blood cells from other nucleated cells (e.g. lining cells). Thus, counts from chamber counters are most accurately considered total nucleated cell counts. Chamber counts are also imprecise, especially at low numbers, and laboratories are increasingly replacing this method with automated cell counters that have higher precision. Automated analyzers are also faster and require less skilled-technologist time than chamber counts. However, they are not accurate at very low levels of cells, as would be seen in a normal cerebrospinal fluid sample. CAP checklist requirement HEM.35452 Acceptable Limits requires that the laboratory define upper and lower limits for counting cells and have a defined action for when the cell count is below the lower limit. In our laboratory, clear CSF samples, which typically have low cell counts, are counted manually using a chamber counter.

Most clinical decision points regarding CSF, pleural fluid, ascites, and synovial fluid are based on absolute WBC counts, so it is valid to report the WBC count with an accompanying WBC-only differential. However, our laboratory, as well as most major U.S. reference laboratories, reports total nucleated cells with a differential that includes non-WBCs. This may be preferred for several reasons. For example, the presence of malignant non-WBCs (mesothelioma or metastatic carcinoma) would not be reported if just WBCs were counted and reported. Using the TNC may also simplify reporting since the laboratory would still have to report out a TNC on any chamber count performed on a low count fluid. It would be cumbersome to report WBC on some samples and TNC on others and to have separate rules about how to perform the differential count in each. This could also lead to confusion with regard to interpreting results. Finally, while the presence of non-WBCs is generally not clinically significant in nonmalignant processes, conditions such as tuberculous pleurisy and rheumatoid pleuritis have a paucity of mesothelial cells in pleural fluid, so excluding these cells from the cell count and differential may omit clinically useful information.

College of American Pathologists. HEM.35452 Acceptable limits. In: Hematology and coagulation checklist. Sept. 17, 2019.

Galagan KA, Blomberg D, Cornbleet PJ, Glassy EF, eds. Color Atlas of Body Fluids. An Illustrated Field Guide Based on Proficiency Testing. Northfield, Ill.: College of American Pathologists; 2006:9–11.

Karcher DS, McPherson RA. Cerebrospinal, synovial, serous body fluids, and alternative specimens. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St. Louis: Elsevier; 2017.

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