Amniotic fluid complement C3 as a marker of intra-amniotic infection.

OBJECTIVE: To determine the value of amniotic fluid (AF) complement C3 as a marker of intra-amniotic infection and to compare complement C3 with other rapid markers of intra-amniotic infection.
METHODS: One hundred four women with singleton gestations, in preterm labor with intact membranes, at 23-35 weeks' gestation underwent transabdominal amniocentesis. Amniotic fluid was analyzed for white blood cell (WBC) count, lactate dehydrogenase (LDH), glucose, Gram stain, and complement C3. Cultures for aerobes, anaerobes, and mycoplasma species also were performed. The median values of complement C3, WBC, LDH, and glucose were compared between the culture-positive and -negative groups. Complement C3 was compared with WBC count, LDH, glucose, and Gram stain for sensitivity, specificity, positive and negative predictive values, and accuracy in the prediction of a positive AF culture. Descriptive statistics, receiver operating characteristic curve, Fisher exact test, and Wilcoxon rank-sum test were used for analysis.
RESULTS: The prevalence of positive cultures was 11.5% (12 of 104). The culture-positive group had a significantly higher median C3 (7.0 mg/dL) than the median C3 (3.0 mg/dL) of the culture-negative group (P < .001). Also, the median values of WBC (1120.5 cells/mm3) and LDH (2697 U/L) were significantly higher and the median glucose (6.5 mg/dL) was significantly lower among women with positive AF cultures than among women with negative AF cultures (WBC=1 cell/mm3; LDH=165 U/L; glucose=45 mg/dL; P < .001). Eleven of the 12 culture-positive cases had a C3 of 5 mg/dL or more, whereas four of the 92 culture-negative cases had a C3 of 5 mg/dL (P < .001). Nine of the 12 culture-positive cases but none of the 92 culture-negative cases had a C3 of 6 mg/dL or more (P < .001). The relative risks of a positive AF culture were 65.27 (95% confidence interval [CI] 9.08, 469.27) and 31.67 (95% CI 10.40, 96.43) times greater among women with AF complement C3 levels of 5 and 6 mg/dL or more, respectively. Depending on the cutoff point used, complement C3 had similar or higher sensitivity, specificity, positive predictive value, and negative predictive value for intra-amniotic infection when compared with WBC count, LDH, glucose and Gram stain.
CONCLUSION: Amniotic fluid complement C3 has value in the diagnosis of intra-amniotic infection in preterm labor with intact membranes. Complement C3 is available readily and compares favorably with other rapid markers of AF infection. This study supports the general concept of fetal inflammatory response to microbial invasion of AF.