CRITERIA FOR FETAL NUCHAL THICKNESS CUTOFF - A REEVALUATION

An attempt has been made to establish a more effective cut-off criteri on for nuchal thickness (NT) and to assess the optimal gestational per iod for the prediction of trisomies 21 and 18. Reference intervals wer e established for NT from the tenth to the 18th week. using either ges tation-specific centiles or the parametric method. The measurements in 47 consecutive trisomy 21 and Is trisomy 18 cases were plotted agains t these intervals. Assaying different cut-off criteria for both the ce ntile and the parametric methods, sensitivities and false-positive rat es for each gestational week were calculated and then compared with th e commonly applied 'two-stepped' cut-off method (3 mm early, 6 mm late r). The parametric method, based on a progressive rise, with +2.5 SD f or the corresponding gestational week as a cut-off value, showed the b est performance (likelihood ratio 38) in the prediction for trisomy 21 . The optimal gestational age was the 12-18 week period, with an overa ll sensitivity of 62 per cent (23/37) for an average false-positive ra te of 0.7 per cent. For trisomy Is, the most effective cut-off was als o +2.5 SD, and 10-13 gestational weeks as the optimal period, achievin g 86 per cent (6/7) sensitivity for a 1.9 per cent false-positive rate .