徐振傑醫師  

Objective:

To evaluate whether the strategy of maternal serum screening for Down syndrome, using alpha fetoprotein (AFP), free beta-human chorionic gonadotropin (b-hCG) and maternal age, developed in occidental countries is applicable in an Asian population.

Methods:

AFP and b-hCG were measured in samples from 23 Down syndrome pregnancies and 1748 singleton unaffected pregnancies between 14 and 22 weeks of gestation. Gestational age-specific normal median values and maternal weight correction formula were established in our own oriental population. Likelihood ratio for Down syndrome in relation to multiple of the median (MoM) level of various analytes were derived from the overlapping Gaussian frequency distribution curves for affected and unaffected pregnancies.

Results:

 Maternal serum AFP and free b-hCG values of Down syndrome pregnancies were significantly abnormal in Asians (median values 0.77 MoM and 2.91 MoM, respectively) which were similar with those of affected pregnancies in Occidentals. The median value of free b-hCG:AFP MoM ratios (2.97) in Down syndrome pregnancies was significantly higher than that of unaffected pregnancies (1.09). The mean of maternal weight during second trimester in Asians (55.2 kg) is significantly lighter than that of white women. At a 5.8% false positive rate, free b-hCG identified 47.8% of Down syndrome pregnancies (likelihood ratio of 8.2 (47.8/5.8)), AFP detected only 13% of the cases (likelihood ratio of 2.2), and free b-hCG:AFP MoM ratios detected 43.5% of the cases (likelihood ratio of 7.4). By using a multivariate risk algorithm in combination of AFP, free b-hCG and maternal age, 56.5% of Down syndrome cases could be detected with a 5.3% false positive rate (likelihood ratio of  10.7).

Conclusion: It seems that second trimester maternal serum screening using AFP and free b-hCG in combination with maternal age could be an effective method in the detection of Down syndrome among Asians.