Results
Participant characteristics
A total of 334 women were potentially eligible, of whom 19 (5.7%) were
excluded as outliers; details on outliers are presented in Appendix S1.
Of the 315 women included in the development cohort, most were white
(93%), had a normal BMI or were moderately overweight, and delivered at
full term (mean gestation 40.4 weeks). Most babies did not require
admission to the neonatal unit (98.1%). On average, overweight women
(≥25 kg/m2) had a significantly higher mean arterial
blood pressure and gave birth to heavier babies than women with a normal
BMI (<25 kg/m2). Fewer than 2% of data were
missing according to any characteristic. Data on participant
characteristics are summarised in Table 1.
Reference interval
development
Trimester-specific reference intervals for CRP are presented in Table 2.
CRP was not associated with maternal age, mean arterial pressure or
ethnicity, but BMI was significantly log-linearly associated with CRP in
all trimesters (see Figure 1 and Appendix S2). CRP was marginally higher
in women pregnant with male babies than female babies, but this finding
was only significant in the second trimester. The statistical conditions
for partitioning the reference intervals for CRP by BMI were either met
or were marginal in all trimesters, using a cut-off of 25
kg/m2, so an overall reference interval was used.
BMI-specific reference intervals are presented in Table 3 and Figure 2.
Diagnostic accuracy
The characteristics of women in the development and evaluation cohorts
were similar (see Table 1). Of the 50 women in the evaluation cohort, 15
(30%) had chorioamnionitis and 35 (70%) did not have any infection.
The median CRP values among women with and without infection were 30.0
mg/L and 8.1 mg/L, respectively (Mann-Whitney test
pdiff<0.001).
The overall diagnostic accuracy of CRP for infection was significantly
higher when using a pregnancy-specific upper reference limit of 18 mg/L,
compared to 7 mg/L (p=0.002) (see Figure 3). The sensitivity and
specificity using the pregnancy-specific reference limit were 80% and
86%, respectively. Summary statistics on diagnostic accuracy for both
reference limits are presented in Table 4.