Antibody-dependent cell-mediated cytotoxicity and phagocytosis of autologous red blood cells in alphamethyldopa-induced haemolysis

Evaluation of the direct antiglobulin (Coombs') test for identifying newborns at risk for hemolysis as determined by end-tidal carbon monoxide concentration (ETCOc); and comparison of the Coombs' test with ETCOc for detecting significant jaundice

OBJECTIVE

First, to determine the sensitivity, specificity, and positive predictive value (PPV) of the direct antiglobulin test (DAT) for significant hemolysis in the neonate, as referenced to end-tidal carbon monoxide, the criterion standard for estimating the rate of hemolysis; and second, to evaluate the predictive value of the two procedures for significant jaundice.

DESIGN

Consecutive term newborns admitted to the nursery of an inner-city university hospital over a 15-week period. DAT screening by the Blood Bank was performed on all. End-tidal carbon monoxide levels were obtained at 12+/-6 and at 24+/-6 hours of age. Infants of nonsmoking mothers whose 12-hour exhaled carbon monoxide level was > or = 95th percentile were defined as having significant hemolysis.

RESULTS

n=660; DAT was positive in 23 (3.5%). Using the 12-hour end-tidal carbon monoxide > or = 3.2 microl/l (> or = 95th percentile) as reference (n=499 nonsmokers), the sensitivity of the DAT was 38.5% (10 of 26) and specificity 98.5% (466 of 473) for the detection of significant hemolysis. The PPV of the DAT for significant hemolysis at 12 hours was 58.8% (10 of 17). For significant jaundice the PPV of end-tidal carbon monoxide was greater than that for DAT (65.4% vs 52.9%), although not statistically so (p=0.25). The negative predictive values were similar.

CONCLUSION

DAT fails to identify over half of the cases of significant hemolysis that are diagnosed by end-tidal carbon monoxide. A neonate with a positive DAT has about a 59% chance of having significant hemolysis. End-tidal carbon monoxide may also provide a more sensitive index for predicting significant jaundice.

Methyldopa-induced autoantibodies against red blood cells

Methyldopa therapy results in the formation of red cell autoantibodies in 10-20% of patients taking the drug for longer than 4 months. These red cell antibodies are true autoantibodies, that is they are directed against an autoantigen on the red blood cell membrane and not against the drug or against a drug-altered antigen. The target membrane antigen is usually within the Rhesus system, although often the antibody specificity cannot be defined. Red cell antibody is usually detectable in the patient's sera as well as on the red cells. The autoantibody is usually a warm reacting IgG antibody. Most patients who develop these autoantibodies do not go on to develop hemolytic anemia in spite of high titres of antibodies on their red cells. In addition, these patients do not tend to develop hemolysis if methyldopa therapy is continued. Rarely patients develop hemolytic anemia which can be severe. Differences in antibody characteristics, including subclass restriction, complement-binding ability, or titre do not explain why some patients with autoantibody hemolyze while most do not. One group of investigators found that hemolyzing patients had IgM on their red cells while those who did not had IgG only. But while this observation could explain why some patients (IgM-sensitized red cells) hemolyze, it does not explain why most patients with IgG-sensitized red cells do not hemolyze. Why the autoantibody forms is not known but some investigators have proposed that the drug may directly affect B or T cells with resulting impairment of immune tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)