NEW YORK (Reuters Health) – When sepsis is suspected in a newborn infant, a complete blood count as a test for infection improves “dramatically” with increasing time over the first few hours of life. If possible, therefore, CBCs to decide on the need for antibiotics should be performed after about 4 hours from birth, researchers advise.

Dr. Thomas B. Newman with the University of California, San Francisco, and colleagues report the findings in the November issue of Pediatrics.

“This study shows that CBCs done in the first hour after birth are not helpful at predicting infection. Decisions about antibiotics in these youngest newborns should be made based on maternal risk factors and newborn signs and symptoms, not the CBC,” Dr. Newman recommended in an email to Reuters Health. “CBC results are much more helpful after 4 hours of age, but still must be considered in combination with clinical factors for optimal decision making.”

The team examined how well components of the CBC identified sepsis in newborns in the first 72 hours after birth in a retrospective study of 67,623 neonates for whom blood cultures were obtained.

The cohort included 245 newborns with positive cultures. The researchers restricted their analysis to the first 24 hours after birth because most of the CBCs were measured in that period.

“Among newborns with infection, mean WBC (white blood cell) counts were 29% lower, mean ANCs (activated neutrophil counts) were 39% lower, and I/Ts (immature-to-total neutrophil counts) were 133% higher than in newborns without infection; platelet counts did not differ significantly,” the team found .

Upon further analysis, only very low WBC counts and ANCs were informative in the first hour. However, there was “a dramatic improvement” in the discrimination of WBC count and ANC during the first 4 hours after birth, according to the report.

Dr. Newman and colleagues say the study has important clinical implications. “If the intent of drawing a CBC is to use that information to make clinical decisions regarding the likelihood of infection, for example, whether to initiate empiric antibiotic therapy, then delaying the CBC for a few hours may be advisable.”

Even then, the likelihood ratios from CBC measurements are modest, so decisions about antibiotic treatment “should remain highly dependent on maternal risk factors and newborn symptoms of infection.”

On the other hand, if the situation seems critical soon after birth, the team suggests that it may be prudent to start antibiotics before 4 hours of age, rather than waiting for results of the CBC.

Reference:

Interpreting Complete Blood Counts Soon After Birth in Newborns at Risk for Sepsis

Pediatrics 2010;129:903-909.