Fetal Acidosis: Likelihood Ratios

Introduction: Intrapartum fetal acidosis, usually secondary to hypoxia, is an indication for immediate delivery of the fetus. Thus, this is the motivation for the current use of continuous fetal heart rate monitoring during labor.

While it is somewhat difficult to actually find the data for sensitivity and specificity of findings on the FHR monitors (many of the studies are from the 70s), I did find the following PPVs from one study. Note that the overall utility of continuous FHR monitoring is at best unclear, and there may be none.

Obstet Gynecol. 1975 Oct;46(4):392-6. PMID: 241036

No accels or decels - PPV for normal pH (>7.25) 90.5%

Accels - PPV normal pH 97.4%

Early decels - PPV normal pH 93%

Variable decels - PPV normal pH 76.7%

Late decels - PPV normal pH 66.6%

FHR 120-160bpm - PPV normal pH 92%

FHR >160bpm - PPV normal pH 80%

FHR <120bpm - PPV normal pH 70%

Beat-to-beat variability was not analyzed in this study.

Am J Obstet Gynecol. 2003 Mar;188(3):820-3. PMID: 12634664

Normal variability - PPV normal pH >=7.0 97%

Decreased variability - PPV normal pH 78%

Decreased variability + var decels - PPV normal pH 87.5%

J Matern Fetal Neonatal Med. 2006 May;19(5):289-94 PMID: 16753769

Normal variability - PPV normal pH >7.2 or Apgar>7 98%

Decreased/absent variability + decels - PPV normal pH 77%

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The likelihood ratios of findings for Fetal Acidosis are listed below. See the left navigation bar to change the display.

Pre-Test Probability (Prevalence): %.	Post-Test Probability (Predictive Value): %.	Switch to display mode.
Note that calculating probabilities from more than one** finding is inherently inaccurate because findings are not independent. (For example, using two positive findings that share a common pathogenesis is likely to overestimate the true probability.)

Findings With Unspecified Accuracy
Finding	Result	+LR	-LR
[ + ] Fetal Heart Rate Monitoring		No accuracy specified.
see text above Study: no study specified.