Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Is there clinical value in doing routine manual band counts to detect infection in newborns, especially since procalcitonin and immature neutrophil counts are available?

A. September 2019—Instrument-generated automated differential counts combine bands and neutrophils in the single category of absolute neutrophil count (ANC). Enumeration of bands thus requires a manual differential. It is well recognized that enumeration of bands is problematic due to poor precision, small sample size, and interobserver variability.^1,2 Moreover, it is insensitive, inefficient, and expensive, and it increases turnaround times. For these reasons, many laboratories have stopped making enumeration of bands a distinct category.

Historically, however, enumeration of bands has been used as a marker of left shift and an indicator of bacterial infection. There are several sepsis screening tools used clinically (see the Surviving Sepsis Campaign website for a partial listing, www.bit.ly/SepsisResources), and while many of the algorithms do not include band percentage as an evaluation criteria, some do. In adults and children older than three months, the white blood cell count and ANC are generally considered better indicators of infection than bandemia.³ In neonatal sepsis evaluation, the immature/total neutrophils (I/T) ratio continues to be used as one of the CBC parameters used clinically. The I/T ratio requires enumeration of bands (see formula below), with an I/T ratio of less than 0.2 being unlikely to have sepsis.

Multiple studies have evaluated the diagnostic value of CBC parameters in neonatal sepsis. While CBC abnormalities such as low white blood cell count, low ANC, and high I/T ratio were associated with culture-positive infections, no CBC-derived index is sufficiently sensitive. A high proportion of culture-negative babies (25–50 percent) may have a high I/T ratio. In most studies, the negative predictive value is higher, with a normal I/T ratio being more helpful in ruling out sepsis, although exhaustion of marrow reserves in critical illnesses can lead to false-negatives.

Several modern CBC analyzers offer a six-part differential, including a category of immature granulocytes (IG). Automated IGs include metamyelocytes, myelocytes, and promyelocytes but do not include bands. Since automation allows better precision and faster turnaround time, assessing left shift using automated IG enumeration, rather than a manual differential, is desirable. A large study evaluated more than 10,000 samples and concluded “that as a way to quantify the leukocyte ‘left shift’ the IG% and IG per μl from an automated differential count on the Sysmex hematology analyzer are comparable to the I/T ratio and absolute band count based on a manual differential count.”⁴

Several studies have evaluated automated immature granulocyte counts and have found them to be of some utility in sepsis assessment.^5,6 The utility of immature granulocytes in neonatal sepsis has also been evaluated in a few studies.^7,8 Overall, both I/T ratio (and thus manual band counts) and automated IG have some utility in neonatal sepsis assessment, but both lack sufficient sensitivity. Automated IGs, however, offer the advantage of better precision, turnaround times, and cost. Given these considerations, if a laboratory wishes to replace manual band counting with automated IG reporting, a dialogue with the neonatologists is advised.