When viewing the complete blood count (CBC) results, what combinations let you know when an infection is bacterial or viral?
Here’s the answer:
The things to look for in the CBC that support the presence of an infectious process are the white blood cell count (WBC) and differential. In non-localized infections of bacterial and viral origin, the total WBC count is elevated in non-immunosupressed patients. The next thing to look at is the differential, which is the percentage of the various types of white blood cells present.
An increase in polymorphonuclear neutrophils, referred to as POLYS or SEGS, is seen in bacterial infections including endocarditis, septicemia, and pneumonia. An immature form of this type of cell, referred to as a BAND, is often also seen, especially in acute infection. The appearance of these younger cells in the circulation is referred to as a SHIFT TO THE LEFT.
A classic example is the emergency patient presenting with right lower quadrant pain, fever, nausea, and vomiting. With a significant increase in SEGS and BANDS, a trip to the OR is pretty much assured. The elevated white count along with types of cells that predominate in this case, support a
diagnosis of acute appendicitis with the threat of rupture.
Viral infections affecting the systemic circulation are often accompanied by an increase in lymphocytes (LYMPHS).
The WBC differential identifies lymphs as reactive or atypical, indicating the response of this cell type to the virus. This occurs in many viral infections including hepatitis, infectious mononucleosis (caused by Epstein-Barr virus) rubella and rubeola.
A frequently encountered scenario is a patient presenting with fever, malaise and swollen lymph nodes. The laboratory report will often note more than 20 percent atypical lymphs (ATL). A mononucleosis screen can then be performed to confirm infection with the Epstein-Barr virus. Cytomegalovirus
(CMV) infection can, at times, present like infectious mononucleosis.
Specific cell types are important in the analysis of cerebrospinal fluid, assisting in the differential diagnosis of bacterial (presence of SEGS) vs. viral (presence of LYMPHS) meningitis.
Some additional points:
The lymphocyte count may also be increased in chronic bacterial infections.
Tuberculosis infection is accompanied by an increase in lymphocytes and monocytes.
Since many infections are localized, changes will not be seen in the CBC. A variety of non-infectious inflammatory conditions, other than infection, can cause increases in the white count, including leukemia and trauma.
The laboratory differential report on a child has different normals reflecting the fact that children have more lymphs and fewer segs in their blood, when compared to adults.
