Table 5.
Effect of serial measurements of cervical length (interpreted as percentage shortening compared with period 1) on the predictive accuracy for preterm birth <32 weeks
| Model | Thresholds * | Predictive accuracy | Relative contribution to DR | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| P1 (mm) |
P2 (%) |
P3 (%) |
P4 (%) |
TP (n) |
FP (n) |
FN (n) |
TN (n) |
DR (%) |
FPR (%) |
PPV (%) |
NPV (%) |
LR+ | LR− | P- value |
P1 | P2 | P3 | P4 | |
| For FPR of 2% | |||||||||||||||||||
| CL on Period 1 < threshold | 26mm | 6 | 8 | 26 | 374 | 19% | 2% | 43% | 94% | 8.95 | 0.83 | Ref | 19% | ||||||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2 < threshold |
20mm | −40% | 9 | 6 | 18 | 335 | 33% | 2% | 60% | 95% | 18.94 | 0.68 | 0.15 | 6% | 33% | ||||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2 or 1→3 < threshold |
15mm | −40% | −60% | 8 | 6 | 13 | 312 | 38% | 2% | 57% | 96% | 20.20 | 0.63 | 0.063 | 6% | 33% | 38% | ||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2, 1→3 or 1→4 < threshold |
20mm | −60% | −70% | −72% | 9 | 5 | 4 | 226 | 69% | 2% | 64% | 98% | 31.98 | 0.31 | 0.016 | 6% | 7% | 25% | 69% |
| For FPR of 5% | |||||||||||||||||||
| CL on Period 1 < threshold | 29mm | 9 | 21 | 23 | 361 | 28% | 5% | 30% | 94% | 5.12 | 0.76 | Ref | 28% | ||||||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2 < threshold |
25mm | −32% | 12 | 17 | 15 | 324 | 44% | 5% | 41% | 96% | 8.92 | 0.58 | 0.17 | 13% | 44% | ||||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2 or 1→3 < threshold |
25mm | −34% | −60% | 9 | 16 | 12 | 302 | 43% | 5% | 36% | 96% | 8.52 | 0.60 | 0.16 | 13% | 42% | 43% | ||
| CL on Periods 1 < threshold OR percentage shortening between Periods 1→2, 1→3 or 1→4 < threshold |
15mm | −34% | −60% | −74% | 10 | 12 | 3 | 219 | 77% | 5% | 45% | 99% | 14.81 | 0.24 | 0.016 | 6% | 41% | 45% | 77% |
CL, cervical length; TP, true positive; FP, false positive; FN, false negative; TN, true negative; DR, detection rate; FPR, false positive rate; PPV, positive predictive value; NPV negative predictive value; LR+, positive likelihood ratio; LR−, negative likelihood ratio; P1, Period 1 (18+0 to 21+6 weeks); P2, Period 2 (22+0 to 24+6 weeks); P3, Period 3 (25+0 to 27+6 weeks); P4, Period 4 (28+0 to 32+0 weeks).
The predictive accuracy of serial measurements of cervical length (interpreted as percentage shortening compared with period 1) for preterm birth <32 weeks was calculated for 4 combinations of serial measurements: Period 1 only, Period 1 + %shortening between period 1→2, Period 1 + %shortening between periods 1→2 and 1→3, and Period 1 + %shortening between periods 1→2, 1→3 and 1→4.
For each of the 4 combinations, the predictive accuracy was calculated using different sets of thresholds. These thresholds were determined using an iteration process that identified the set of thresholds that is associated with the highest detection rate for each of the following target levels of false positive rate: 2% and 5%.