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Letter to ‘The diagnostic value of fetal fibronectin testing in clinical practice’

Dear Editor, I was interested in reading the paper by Cornelissen et al. that was published in the March 2020 edition of the Journal of Obstetrics and Gynaecology Research. The purpose of the authors was to evaluate the clinical management to withhold treatment for preterm labor in symptomatic women with an intermediate cervical length and negative fetal fibronectin (fFN) testing. Pregnant women with a gestational age between 23 to 34 weeks, with the presence of regular uterine contractions accompanied by a cervical length between 15 and 30 mm and intact membranes, who underwent fFN testing were included to obtain the diagnostic value of fFN testing for preterm delivery within 7 days. Fetal fibronectin testing has an extremely high negative predictive value (NPV) (100%), sensitivity (100%) and likelihood ratio negative (LR−) (0.0) for delivery within 7 days, in singleton and multiple pregnancies. However, specificity (64%), positive predictive value (PPV) (10%) and LR+ (2.7) of fFN testing in singleton pregnancies are low. Blood present on the fFN sample does not affect the reliability of the fFN test; the NPV remains 100%. They concluded that administration of corticosteroids and tocolytics can safely be withhold. Furthermore, blood on the fFN sample does not change the reliability of the fFN test. First, it is crucial to know that reproducibility (reliability, precision and repeatability) and validity (accuracy) are two completely different methodological issues of diagnostic value. Without assessing reliability, we cannot talk about diagnostic value. Second, sensitivity, specificity, PPV, NPV as well as likelihood ratios (likelihood ratio positive [ranging from 1 to infinity; the higher the LR+, the more accurate the test], likelihood ratio negative [ranging from 0 to 1; the lower the LR−, the more accurate the test]) are among the estimates to assess validity (accuracy) of a diagnostic test and have nothing to do with reliability. It should be noted that, due to the limitation of reported values for accuracy (e.g. sensitivity and specificity are generally used for public health purposes and limited in clinical practice. PPV extremely depends on the prevalence of the outcome); diagnostic added value of a test should be considered to judge about clinical importance of a diagnostic test. The point is reported estimates as in this study can be acceptable (high negative predictive value [100%], sensitivity [100%] and LR− [0.0]); however, considering the rest of validity estimates (specificity [64%], PPV [10%] and LR+ [2.7]), our final decision can easily be changed. Moreover, none of the above mentioned estimates can assess reliability. Third, receiver operative characteristic curve is usually used to assess diagnostic accuracy (discrimination) of a diagnostic model. However, for clinical purposes, reporting diagnostic added value is crucially important. The reason is all validity estimates can be acceptable but diagnostic added value may be negligible. Any conclusion in reliability and validity analyses needs to be supported by the methodological and statistical issues mentioned above. Otherwise, misinterpretation cannot be avoided.