To evaluate the relationship between platelet counts and the platelet function analyzer-100 closure times (CTs) in neonates with thrombocytopenia, and to determine what other factors significantly affect CTs.

In a single institution prospective cross-sectional study, blood samples from neonates with platelet counts <150 × 10⁹/L were tested on the platelet function analyzer-100 with CT-collagen/epinephrine (CT-Epi) and CT-collagen/adenosine diphosphate (CT-ADP) cartridges.

The mean platelet count was 95 ± 28 × 10⁹/L for 48 infants with a mean gestational age 30.9 ± 5.3 weeks and median postnatal age of 5 (3-18) days. No association was evident between CT-Epi and platelet count. However, the CT-ADP was prolonged in many (but not all) infants with platelet counts <90 × 10⁹/L. Among infants <32 weeks gestational age, we found a moderate negative correlation between CT-ADP and platelet count (r = −0.54, P = .0045). The negative correlation was strongest in infants <32 weeks and <10 days old (r = −0.8, P = .0017). Other variables examined (hematocrit, infection, Score of Neonatal Acute Physiology II) did not have a significant effect on CT-ADP in a linear regression model.

Platelet counts <90 × 10⁹/L are associated with prolonged CT-ADP times in some but not all infants. Gestational and postnatal age-related differences in platelet function account for some of this variability. The predictive value of CT-ADP on neonatal bleeding risk remains to be studied.