Guidelines for the management of spontaneous preterm labor: identification of spontaneous preterm labor, diagnosis of preterm premature rupture of membranes, and preventive tools for preterm birth

Dear Editor, With great interest I read the recently published guidelines for the management of spontaneous preterm labor [1]. I was delighted to see that unlike the previous guidelines published in 2006 [2], these new ones also take into consideration the diagnostic marker insulin-like growth factor binding protein-1 (IGFBP-1) that I have worked with since the early 80s. However, I would like to bring the readers’ attention to some errors and points that may be misleading regarding evaluation of IGFBP-1 as a marker of ruptured fetal membranes (ROM) and in comparing it with placental α microglobulin-1 (PAMG-1). Firstly, human IGFBP-1 is a well characterized protein since more than 20 years [3,4]. Its synthesis by the liver and decidua, and levels in amniotic fluid and other body fluids have been thoroughly examined in all stages of pregnancy [5,6] and the data have been published in peer-reviewed journals. Meanwhile, the data available on PAMG-1 is more limited and partly confusing. In the most often cited papers regarding the PAMG-1 levels in amniotic fluid, blood and other body fluids [7–9], the values are quite different from those reported in the guidelines. This makes comparison between IGFBP-1 and PAMG-1 difficult. IGFBP-1 has been used as a marker of ROM since the mid 90s (Actim PROM test). Since then, several studies have consistently shown that this test identifies membrane rupture with high accuracy. Unfortunately, many of these studies were omitted in the analysis presented in Table I of the guidelines comparing the performance of the different tests [10–12]. As a consequence, the sensitivity and specificity of the IGFBP-1 test remain underestimated. For example, the lowest sensitivity (74%) is from a study by Lockwood 1994 using a quantitative radioimmunoassay with frozen samples in the laboratory with a different detection limit and assay conditions from the current IGFBP-1 based bed-side PROM test [13]. Secondly, the guidelines state that the detection limit of PAMG-1 with Amnisure ROM test (5 ng/ml) is lower than the detection limit of IGFBP-1 with Actim PROM test (25 ng/ml). This comparison is irrelevant, since the quoted levels of PAMG-1 protein in amniotic fluid (2000−25,000 ng/ml) are clearly lower than the known levels of IGFBP-1 (10,500−350,000 ng/ml [14]), which naturally calls for a need of a lower detection limit. In the guidelines the lowest level of IGFBP-1 is quoted to be 27 ng/ml in early pregnancy. Such low levels have not been reported at pregnancy weeks clinically relevant for diagnosis of ROM [15]. Thirdly, the sensitivity and specificity of any test has to be interpreted in the clinical context. The methods used for estimation of the accuracy and reliability of the PAMG-1 test compared to the IGFBP-1 test are questionable for several reasons. For example, samples of pure blood-free amniotic fluid obtained during intra-operative amniocentesis at cesarean section were diluted with 0.9% saline and serial dilutions were tested using both tests [16,17]. The study design does not correspond to the bed-side situation where amniotic fluid is contaminated by vaginal discharge or other possible fluids like urine, semen or blood that may affect the test result causing false positives, if the test is too sensitive. A high rate of positive PAMG-1 test results has been found among patients with intact membranes and labor at term [18] and in patients with a short cervix [19]. This data has not been considered when analyzing the specificity of PAMG-1 test. Also, the publication on the intra-amniotic dye test and its comparison with the PAMG-1 test is a congress abstract only, with no information on the numbers of patients or the study design [20]. Finally, IGFBP-1 test results have repeatedly been shown to be unaffected by the presence of blood [10,21,22]. Indeed, the monoclonal antibody used in the Actim PROM test does not recognize the highly phosphorylated IGFBP-1 which is the predominant isoform in maternal and fetal blood and decidua [4]. Since blood may be present in approximately 25% of cases with suspected PROM, this information is critical in order to estimate the accuracy and clinical usefulness of the marker. Suspected rupture of membranes in the presence of bleeding is the most challenging situation in the clinic, since the therapeutic measures differ depending on whether the membranes in such a case are intact or not. Yet, no information is available on the accuracy of the PAMG-1 test in patients with suspected membrane rupture and bleeding since patients with bleeding have systematically been excluded in PAMG-1 clinical studies, suggesting that PAMG-1 test cannot be used in such challenging cases. The statement that presence of blood up to 50% does not interfere with the PAMG-1 test result is only based on a conference poster, reporting serial dilutions of peripheral blood in 0.9% saline in vitro [23]. Again, the study design is not equivalent to the clinical situation where amniotic fluid in cervicovaginal swab sample is mixed with vaginal secretion and other possible contaminants. Also, this high rate of positive Amnisure results in the presence of blood raises a question on the validity of the reported range in the maternal blood (0.5–2 ng/ml) that should not react in a test with a detection limit of 5 ng/ml. Considering all the points above, the currently available data does not unequivocally support the superiority of the PAMG-1 test as compared with the IGFBP-1 test.