Serum creatine kinase is not a reliable indicator of ectopic pregnancy

It has been reported that elevation in creatine kinase may be used as an indicator of ectopic pregnancy as a consequence of trophoblast invasion of the smooth muscle layer of the Fallopian tube. In this study, we attempt to verify this observation and establish the sensitivity and positive predictive value of the test.

Serum creatine kinase (MM-fraction) was obtained from patients presenting with possible ectopic pregnancy prior to surgical intervention. Ectopic pregnancies were confirmed by laparoscopy. Noted was whether the gestation was ruptured or unruptured, its location in the tube, and estimated cross-sectional tubal distention. Controls consisted of patients with first-trimester pregnancies simulating ectopic pregnancy, i.e., patients having a positive pregnancy test, complicated by vaginal bleeding and/or abdominal pain.

The distribution of creatine kinase values for both patients with ectopics versus control are positively skewed, with a mode of 10-19 U/L. The mean creatine kinase for ectopic pregnancies was 62.3 U/L, S.D. = 63.1 U/L. For controls, the mean creatine kinase = 40.8 U/L, S.D. = 30.1 U/L. The difference in these means is significant (p = 0.04), but weakly so. In addition, for ectopic gestations there was no significant difference in creatine kinase values with regard to degree of tubal distention, gestation location, or whether rupture had occurred, thereby limiting the clinical value of this test.

Although the mean value of serum creatine kinase is statistically significantly higher in ectopic pregnancy relative to controls, the distribution curve for creatine kinase values for ectopics is broad, with much overlap with the control curve. Hence, the sensitivity and positive predictive value for this test is poor.