ObjectiveTo determine whether neonatal lymphocyte or nucleated red blood cell (RBC) counts can be used to date fetal neurologic injury.MethodsSingleton, term infants with hypoxic-ischemic encephalopathy, permanent neurologic impairment, and sufficient laboratory data were divided into two groups: infants with preadmission injury, manifested by a nonreactive fetal heart rate (FHR) pattern from admission until delivery; and infants with acute injury, manifested by a normal FHR pattern followed by a sudden prolonged FHR deceleration. Lymphocyte and nucleated RBC values were compared with published high normal counts for normal neonates: 8000 lymphocytes/mm3and 2000 nucleated RBCs/mm3.ResultsThe study population consisted of 101 neonates. In the first hours of life, lymphocyte counts were elevated among injured newborns, and then the counts rapidly normalized. Brain-injured neonates were 25 times more likely to have a lymphocyte count greater than 8000 than were normal neonates (54 [62%] of 87 versus 6 [7%] of 84; odds ratio 25.5; 95% confidence interval 8.8, 80.1;P< .001). The mean lymphocyte count tended to be higher in the pread-mission-injury group than in the acute-injury group. In comparison, nucleated RBC values were not correlated as strongly with neonatal hours of life; nucleated RBC counts tended to be higher and persist longer among neonates with preadmission injury than among those with acute injury.ConclusionCompared with normal levels, both lymphocyte and nucleated RBC counts were elevated among neonates with fetal asphyxial injury. Both counts appear to be more elevated and to remain elevated longer in newborns with preadmission injury than in infants with acute injury. However, the rapid normalization of lymphocyte counts in these injured neonates limits the clinical usefulness of these counts after the first several hours of life.