ABO incompatibility affects up to 15% of mother and fetus binomial. Hemolysis secondary to ABO incompatibility is an important differential diagnosis among newborns presenting with jaundice. Most studies evaluating risk factors for newborn hemolysis due to ABO incompatibility date from almost 40 years ago and literature lacks reports describing the laboratorial factors predictive of this disease after the advent of modern immunohematological reagents, which are more sensitive.

To determine the clinical and laboratorial factors associated with the occurrence of overt hemolysis due to ABO incompatibility among Brazilian newborns.

This was a nested case control study stemming from a cohort of 4,122 newborns of three different Brazilian hospitals. Mothers and newborns were ABO typed using conventional tube method and direct antiglobulin test (DAT) was also performed in tube using polyspecific anti human globulin. The occurrence of jaundice secondary to ABO incompatibility was retrieved from the medical files. Diverse clinical and laboratorial factors were associated with the endpoint (ABO incompatibility disease) using Chi-square test for categorical variables and t-test for continuous variables. Multivariable analysis was performed using logistic regression. A p-value less than 0.05 was considered significant.

Among the 4,122 newborns 44 had the diagnosis of hemolytic disease due to ABO incompatibility. The incidence of this complication was 7.9% considering only the situations in which there was ABO incompatibility between mother and fetus. Positive DAT, group O mother and group A newborn were significantly associated with the occurrence of clinical symptoms of hemolysis and this association was kept in multivariable model (p < 0.001 for the three variables). The presence of intra-uterine growth arrest, use of corticosteroids or antibiotics by the mother, type of birth (C-section or normal) and mother self-declared race were not associated with the occurrence of overt hemolysis secondary to ABO incompatibility. DAT presented 65.85% sensitivity, 96.28% specificity, 16.9% positive predictive value and 99.6% negative predictive value for the diagnosis of ABO incompatibility hemolysis. There were no cases of positive DAT in situations other than mother group O and newborns group A or B. The hemoglobin presented by the newborns right after birth was significantly lower when mothers were of group O and newborns were of group A (p < 0.001).

Positive DAT, mother of group O and newborn of group A are independent factors associated with the occurrence of hemolysis due to ABO incompatibility of clinical relevance. With the improvement of the immunohematological reagents, positive DAT poses as an important diagnostic tool for the diagnosis of this complication, since the negative predictive value of the test is very high.