What is the role of CBC count in the workup of neonatal sepsis?

Updated: Jun 13, 2019
  • Author: Nathan S Gollehon, MD, FAAP; Chief Editor: Muhammad Aslam, MD  more...
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A CBC and differential may be ordered serially to determine changes associated with the infection (eg, thrombocytopenia or neutropenia) or to monitor the development of a left shift or changes in the ratio of immature to total neutrophils, although the sensitivity and specificity of these markers is low. Serial monitoring of the CBC may be useful in aiding the differentiation of sepsis from nonspecific abnormalities due to the stress of delivery.

Platelet count

The platelet count in the healthy newborn is rarely lower than 100,000/µL in the first 10 days of life (normal, ≥150,000/μL). Thrombocytopenia (platelet count < 100,000/µL) may be a presenting sign of neonatal sepsis and can last as long as 3 weeks; 10%-60% of infants with sepsis have thrombocytopenia. [42]  However, thrombocytopenia is an insensitive and nonspecific finding as well as a late indicator of serious bacterial infection, making its utility in the initial workup of neonatal sepsis questionable.

Because of the appearance of newly formed platelets, mean platelet volume (MPV) and platelet distribution width are significantly higher in neonatal sepsis after 2-3 days of life. These measures may assist in determining the cause of thrombocytopenia. However, owing to the myriad of causes of thrombocytopenia and its late appearance in neonatal sepsis, the presence of thrombocytopenia does not aid the diagnosis of neonatal sepsis.

White blood cell counts and ratios

Although white blood cell (WBC) counts and ratios are more sensitive for determining sepsis than platelet counts are, they remain very nonspecific and have a low positive predictive value. Normal WBC counts may be initially observed in as many as 50% of cases of culture-proven sepsis. Infants who are not infected may also demonstrate abnormal WBC counts related to the stress of delivery or to any of several other factors. A low WBC count (< 5,000/µL) is associated with a higher likelihood ratio for sepsis than an elevated WBC count (>20,000/µL). [43]

A differential may be of use in diagnosing sepsis; however, these counts are largely dependent on the laboratory technician performing them. The total neutrophil count (polymorphonuclear cells [PMNs] and immature forms) is slightly more sensitive for determining sepsis than the total leukocyte count (percent lymphocyte + monocyte/PMNs + bands), although the overall likelihood ratio remains low. 

Abnormal neutrophil counts at the time of symptom onset are observed in only two thirds of infants; therefore, the neutrophil count does not provide adequate confirmation of sepsis. Neutropenia is also observed with maternal hypertension, severe perinatal asphyxia, and periventricular or intraventricular hemorrhage.

Neutrophil ratios may have some limited utility in the diagnosis of neonatal sepsis. The immature-to-total (I/T) ratio is the most sensitive (60%-90%). All immature neutrophil forms are counted. The maximum acceptable I/T ratio for excluding sepsis in the first 24 hours is 0.16. In most newborns, the ratio falls to 0.12 within 60 hours of birth. Because elevated I/T ratios may be observed with other physiologic events, their positive predictive value is limited. In addition, the specificity of the I/T ratio is only 50%-75%, limiting its clinical usefulness. 

The utility of the CBC increases after the first 4 hours of age, with the WBC count findings, I/T ratio, presence of thrombocytopenia, and low absolute neutrophil count (ANC) all having significantly improved likelihood ratios in this later timeframe. Delaying the CBC until 4 hours or later may be prudent if the intent of its use is to make decisions regarding the likelihood of infection. In this context, decisions about antibiotic treatment should largely be based upon clinical findings and maternal risk factors. [43]

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