Criteria for reflex peripheral smear review in infants

Criteria for peripheral smear review are designed to include those samples with results outside the reference interval and can be more extreme based on what is considered to have clinical utility. However, we are unaware of previous studies that reported the distributions of various complete blood cell count (CBC) parameters in infants. In the following study we reviewed screening CBC results of 692 infants aged 9-15 months in order to determine the proportion of peripheral smear reviews recommended according to consensus criteria and that after adjusting for the observed distributions of the various parameters. According to consensus criteria the recommended reflex peripheral smear review rate was 39.7% (95% CI 36.1-43.4) whereas after adjustment for the observed distributions, the rate fell to 5.6% (95% CI 3.9-7.3) (p < 0.001). The major reasons for the difference in rates were the high proportion of infants with an absolute lymphocyte count > 7 × 10⁹/L (17.5%), the presence of a plus one blast flag (4.3%), and a large unstained cell count of ≥ 5% (26.2%) (equivalent to + 1 atypical flag). We found that international consensus criteria for reflex peripheral smear review results in a very high peripheral smear review rate in well infants, and might be inappropriate.