In the United States there is an epidemic of hypertensive disorders in pregnancy, with 16% of pregnant people being diagnosed with preeclampsia, gestational hypertension, chronic hypertension, preeclampsia superimposed on chronic hypertension, HELLP, or eclampsia.1 Preeclampsia with severe features increases the maternal risk for stroke, pulmonary edema, kidney injury, abruption, and fetal and maternal death. Preeclampsia also increases the fetal risk for growth restriction, oligohydramnios, and preterm birth.
Angiogenic factors and the pathophysiology of preeclampsia—From bench to bedside
The pathophysiology of preeclampsia is not fully characterized, but a leading theory is that placental ischemia causes increased placental production of anti-angiogenesis factors and a decrease in placental production of pro-angiogenesis factors.2-4 Clinical studies support the theory that preeclampsia is associated with an increase in placental production of anti-angiogenesis factors, including soluble fms-like tyrosine kinase 1 (sFlt-1) and soluble endoglin, and a decrease in the placental production of pro-angiogenesis factors, including placental growth factor (PlGF).5-15
The US Food and Drug Administration (FDA) has recently approved an assay for the measurement of sFlt-1 (Brahms sFlt-1 Kryptor) and PlGF (Brahms sFlt-1 Kryptor) (Thermo Fisher Scientific; Waltham, Massachusetts).16 This editorial focuses on the current and evolving indications for the measurement of sFlt-1 and PlGF in obstetric practice.
FDA approval of a preeclampsia blood test
The FDA approval of the tests to measure sFlt-1 and PlGF is narrowly tailored and focused on using the sFlt-1/PlGF ratio to assess the risk of progression to preeclampsia with severe features within 2 weeks among hospitalized patients with a hypertensive disorder in pregnancy with a singleton pregnancy between 23 weeks 0 days (23w0d) and 34w6d gestation.16 The test is meant to be used in conjunction with other laboratory tests and clinical assessment. The FDA advises that the test results should not be used to diagnose preeclampsia, nor should they be used to determine the timing of delivery or timing of patient discharge.16 The sFlt-1 and PIGF measurements are both reported as pg/mL, and the sFlt-1/PlGF ratio has no units.
The FDA approval is based on clinical studies that demonstrate the effectiveness of the test in predicting the progression of a hypertensive disorder in pregnancy to preeclampsia with severe features within 2 weeks of testing. In one study, the sFlt-1/PlGF ratio was measured in 556 pregnant patients with a singleton pregnancy who were between 23w0d and 34w6d gestation and hospitalized with a hypertensivedisorder in pregnancy without severe features at study enrollment.15 Those patients receiving intravenous heparin were excluded because of the effect of heparin on sFlt-1 levels. Participants’ mean age was 31.7 years, and their mean gestational age was 30w3d. The patients’ mean body mass index (BMI) was 34.2 kg/m2, with mean maximal blood pressure (BP) at enrollment of 159 mm Hg systolic and 95 mm Hg diastolic.
In this cohort, 31% of enrolled patients progressed to preeclampsia with severe features within 2 weeks. At enrollment, the median sFlt-1/PlGF ratio was greater among the patients who progressed to preeclampsia with severe features than among those who did not have progression to preeclampsia with severe features (291 vs 7). An elevated sFlt-1/PlGF ratio (determined to be a ratio ≥ 40) predicted that patients would progress to severe preeclampsiawith severe features—with positive and negative predictive values of 65% and 96%, respectively. Among the subgroup of patients with a history of chronic hypertension, an sFlt-1/PlGF ratio ≥ 40 had positive and negative predictive values of 59% and 94%, respectively. Focusing the analysis on patients who self-reported their race as Black, representing 30% of the cohort, the positive and negative predictive values for a sFlt-1/PlGF ratio ≥ 40 were 66% and 99%, respectively.15
Receiver-operating curve analyses were used to compare the predictive performance of sFlt-1/PlGF measurement versus standard clinical factors and standard laboratory results, including systolic and diastolic BP; levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and creatinine; and platelet count.15 The area under the curve for predicting progression to preeclampsia with severe features was much greater for the sFlt-1/PlGF test (0.92) than for systolic (0.67) and diastolic BP (0.70), AST level (0.66), ALT level (0.61), creatinine level (0.65), and platelet count (0.57).15 These results demonstrate that measuring sFlt-1/PlGF ratios is a much better way to predict the progression of preeclampsia to severe disease than measuring standard clinical and laboratory results.
Patients with a sFlt-1/PlGF ratio ≥ 40 had higher rates of adverse maternal outcomes including severe hypertension, abruption, stroke, eclampsia, pulmonary edema, thrombocytopenia, low platelets, and/or coagulation disorder, than those patients with a ratio < 40, (16.1% vs 2.8%, respectively; relative risk [RR], 5.8; 95% confidence interval [CI], 2.8 to 12.2).15 Adverse fetal and neonatal outcomes (including fetal death, small for gestational age and early delivery due to progression of disease) were more common among patients with a sFlt-1/PlGF ratio of ≥ 40 (80% vs 26%; RR, 3.1; 95% CI, 2.5–3.8).15 Many other studies support the hypothesis that the sFlt-1/PlGF ratio is predictive of adverse outcomes among patients with hypertensive disorders in pregnancy.6-15
Applying the bottom-line study findings. Patients with a hypertensive disorder in pregnancy and a sFlt-1/PlGF ratio < 40 have a low risk of progressing to preeclampsia with severe features over the following 2 weeks, with a negative predictive value of 96%. The remarkably high negative predictive value of a sFlt-1/PlGF ratio < 40 will help obstetricians generate a care plan that optimizes the use of limited health care resources. Conversely, about two-thirds of patients with a hypertensive disorder in pregnancy and a sFlt-1/PlGF test ≥ 40 will progress to preeclampsia with severe features and may need to prepare for a preterm delivery.
Continue to: Clinical utility of the sFlt-1/PlGF ratio in obstetric triage...