Defining critical antibody titre in column agglutination method to guide fetal surveillance

A critical anti D antibody titre, defined for the conventional tube method of Indirect Coomb's test (ICT), when employed in the more sensitive column method could result in unnecessary referrals and frequent obstetric doppler scans. This study aimed to compare anti D titres by tube and column method in antenatal mothers, to assess their correlation with fetal anemia and to determine a critical titre for the column method.

Forty six antenatal mothers with anti D antibody were included in the study. Antibody titration was performed by serial twofold dilution of serum by both column and tube method and were correlated with middle cerebral artery peak systolic velocity (MCA PSV) measurement by Doppler ultrasonography. Receiver operating curve (ROC) was used to determine the cut-offs for critical titre by tube and column method in predicting fetal anemia.

Column method had a median titre 3 fold higher than tube method. There was a significant association between fetal anemia by USG with median critical titres determined for both column (p = 0.031) and tube method (p = 0.016). ROC analysis showed the cut off for critical titres in column method as 64 with 90 % sensitivity, 72.7 % specificity and 75.38 % accuracy.

The use of critical titre for anti D antibody, defined for the tube method, when applied to the column agglutination method would lead to increased referrals to specialized fetal medicine centres. Rather, an Anti D titre of 64 by column method can predict the likelihood of fetal anemia and should be considered as the critical titre to guide patient referrals.