Hematocrit: an uncontrollable variable in newborn screening?

Introduction: dried blood spots (dbs) are valuable samples for the newborn screening laboratory. Cut-off values for neonatal screening determinations are based on an average hematocrit value (55 %). Newborns that exceed the cut-off value of the determinations are cited again for a confirmatory sample collection. Objective: to develop a laboratory technique that allows knowing the value of the hematocrit in the dbs samples of neonatal screening and to study the impact of the hematocrit variable on the results of neonatal screening. Material and methods: 1124 samples from two neonatal screening laboratories were analyzed. The hematocrit in the DBS samples was estimated using immunoreactive trypsin (IRT), and 17-hydroxyprogesterone (17-OHP) were determined. The results were evaluated with and without hematocrit correction taking into account the estimated value. Results: 183 samples (without correction) exceeded the cut off value proposed by each laboratory: 49 for tsh, 44 for irt, and 90 for 17 OHP. Using the hematocrit estimation, 18 (36.7 %) newborns exceeded the TSH cut-off value, 25 (56.7 %) in the IRT measurement and 39 (43.3 %) in 17OHP determination. If the correction of the packed red cell had been used in the DBS samples, only 44.8% of the 183 newborns would have had to be cited again. Discussion: the estimation of the hematocrit would allow the correction of the volume in the dbs samples. We suggest the use of hematocrit estimation for samples that exceed the cut-off value. By lowering the percentage of requests for new samples we would avoid the anguish that it causes in the family.