Hong Kong Journal of Emergency Medicine

Volume 11 Number 1, January 2004

Should differential white cell counts be reported as percentages or absolute counts in patients with Severe Acute Respiratory Syndrome?

TH Rainer, PA Cameron, DV Smit

Background: Severe acute respiratory syndrome (SARS) is associated with a lymphopenia, thrombocytopenia and neutrophilia and suspected cases may be admitted to hospital on the basis of such abnormalities. Laboratories may report changes as percentages or absolute counts. Objective: To investigate whether absolute or percentage differential counts were more predictive of patients with SARS pneumonia. Design: Prospective observational study. Setting: SARS clinic of an emergency department in Hong Kong. Subjects and methods: Whole blood and differential counts were performed on 506 patients presenting to a SARS screening clinic. Ninety-six patients subsequently developed SARS pneumonia. Results: Sixty-nine patients had abnormal lymphocyte absolute counts on first attendance at clinic of which 37 (54%) developed SARS pneumonia. This compared with 142 subjects with abnormal percentage lymphocyte values of which 50 (35%) developed SARS pneumonia. The area under the ROC curve for absolute lymphocyte counts is 0.851 (95%CI 0.816 to 0.881) and for percentage lymphocytes is 0.736 (95%CI 0.694 to 0.775). The area under the ROC curve for absolute monocyte counts is 0.535 (95%CI 0.489 to 0.580) and for percentage monocytes is 0.635 (95%CI 0.591 to 0.678). The area under the ROC curve for absolute neutrophil counts is 0.591 (95%CI 0.546 to 0.636) and for percentage neutrophils is 0.703 (95%CI 0.660 to 0.744). Conclusion: Reporting absolute rather than percentage values for differential leucocyte counts are more accurate predictors of SARS pneumonia. (Hong Kong j.emerg.med. 2004;11:12-15)

Keywords : Prediction, sensitivity, Severe Acute Respiratory Syndrome, specificity

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