Laboratory markers to discriminate between bacterial and viral infections

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Peltola et al collate more than one decade of experience of concomitant measurement of peripheral white blood cell (WBC) count and serum C-reactive protein (CRP) level to distinguish bacterial versus viral infection. Their data show that high WBC is a good marker for pneumococcal but not staphylococcal infection and that CRP adds little clinical value to WBC in distinguishing bacterial from viral infection. Pursuit of the holy grail of laboratory markers continues. The clinical constellation of the context and history of illness, findings on examination and simple laboratory tests together trump any individual test. The study’s findings regarding Staphylococcus aureus remind us of important clinical concepts. For example, infection due to exotoxin- or endotoxin-producing organisms (S. aureus, S. pyogenes, gram-negative bacilli) frequently is associated with normal or low total WBC count with shift to immature forms. CRP is a measure of tissue necrosis or inflammation regardless of cause – most useful to monitor response to interventions or natural history of disease.