A systematic review including 44 studies with a total of 3261 samples was abstracted in DARE. Overall accuracy was 2980 samples out of 3261 (91.4%). Individual study estimates of accuracy ranged from 31.8 to 100%. After the exclusion of small studies and unsuitable samples, accuracy was 2919/3078 (94.8%). The pooled sensitivity was 95.4% (95% CI 90.6 to 97.8) and the pooled specificity 98.6% (95% CI 96.4 to 99.5). Accuracy was higher in the first trimester (90.8%, 218/240) than in the second (85.0%, 350/421) or third trimesters (85.3%, 232/272); p=0.041. Studies using free DNA reporting significantly greater accuracy than DNA or RNA from foetal cells. The greatest accuracy was obtained using free DNA in maternal plasma (2293/2377 samples; 96.5%, 95% CI 95.6 to 97.2) and free DNA from maternal serum (394/410 samples; 96.1%, 95% CI 84.1 to 96.2). The accuracy of free foetal DNA in maternal blood was lower (90/98 samples; 91.8%, 95% CI 84.1 to 96.2). The use of DNA or RNA from foetal cells in maternal blood showed poorer accuracy: 42 samples out of 62 (67.7%, 95% CI: 54.5, 78.7) and 100 samples out of 131 (76.3%, 95% CI: 68.0, 83.1), respectively. Alloimmunisation had no effect on accuracy: foetal Rh type was correctly diagnosed in 719 (91.8%) of the 783 alloimmunised patients.
Comment: The quality of evidence is downgraded by review quality (limited data about the individual studies and review process) and by inconsistency (heterogeneity was not formally assessed).