A method of measuring the severity of a series of cases of hemolytic disease of the newborn

1. The hemoglobin concentration in the cord blood of infants affected withhemolytic disease is very closely related to their chance of survival. If the hemoglobin concentration is plotted against the chance of survival, a sigmoid curve isobtained. Probit analysis can be applied and the results of treatment can then besummarized in terms of the severity of the cases treated. For example, in thepresent series the cord hemoglobin concentration corresponding to a 50 per centchance of survival was 8.09 Gm./100 ml.

2. Application of probit analysis makes it possible to compare results obtainedin different centers, for due allowance is made for any difference in severity in thecases included in the different series.

3. In the present series, despite uniform treatment (exchange transfusion),over-all mortality fell from 37 percent in 1947 and 1948 to 3 per cent in 1950. It isshown that this can be entirely explained by a decrease in the number of severelyaffected infants.

4. Predictions of survival based on the cord hemoglobin concentration and cordbilirubin concentration combined are not significantly better than predictionsbased on cord hemoglobin concentration alone.

5. Infants with positive direct Coombs tests whose cord hemoglobin concentrations exceed 15.5 Gm./100 ml. have some risk (approximately 1 in 12) of developing kernicterus, if untreated.

Submitted on March 29, 1951 Accepted on April 18, 1951