The utility of CBC results in initial fever investigations

Different infections have diverse effects on the blood cells, whereas other causes of febrile illness may have insignificant or no effects on these cells. One of the largest uses of CBC test results is therefore in fever investigations.

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A CBC is a cost-efficient analysis compared with more specific test, and therefore readily available in laboratories worldwide. Including a complete blood count together with other elements of initial patient assessment, can provide early indications for possible diagnosis and support decision-making for further testing and treatment plan (Table 1).
Compared with more specific tests such as dengue PCR and NS1 antigen tests, malaria immunochromatographic test, or blood cultures that can take days to results, CBC test results can be provided within minutes. The results help suggest a likely diagnosis, while waiting for confirmatory tests in early fever investigations.

Table 1. Usual findings in assessment of patients suffering from febrile illness upon return from travel in tropical countries

Considering classic fever as a temporary elevation of body temperature, the cause can be other than an infection, such as the result of immunization, heat exhaustion, or medication that may have little or no impact on the CBC results. In COVID-19 vaccination, for example, it is important for general practitioners to differentiate between post-immunization fever and onset of COVID-19 virus infection.

One of the most relevant parameters to monitor COVID-19 patients has proven to be the lymphocyte count, as these cells are the subgroup of leukocytes involved in virus infections and their count has shown to be associated with severity of infection. A lymphocyte count in the upper range or above normal values is indicative of an immune response in-line with the infection, whereas a sudden drop could be an indication of that the patient is entering into an acute phase of the disease. Another important parameter in management of COVID-19 patients is the platelet count. Many serious cases have shown a platelet count in the lower range or below normal ranges. A CBC test, including a WBC differential count, has therefore been suggested to be included in early fever investigations in regions affected by COVID-19.
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