Accuracy of Mean Arterial Pressure and Blood Pressure Measurements in Predicting Pre-Eclampsia: Systemic Review and Meta-Analysis
Cnossen JS, Vollebregt KC, de Vrieze N, et al.
BMJ 2008;336:1117–1120..
Diagnostic Accuracy of Urinary Spot Protein: Creatinine Ratio for Proteinuria in Hypertensive Pregnant Women: Systemic Review
Côté AM, Brown MA, Lam E, et al.
BMJ 2008;336:1003–1006..
Mean Arterial Pressure and Prediction of Pre-Eclampsia
Walsh CA, Baxi LV.
BMJ 2008;336:1079–1080..
Preeclampsia affects about 5% of all pregnancies and is not confined to any population group. Its diagnosis and treatment is life saving as reflected in fatality rates from eclampsia, ranging from 5% in developing countries to less than 1% in developed countries. In the United Kingdom, it remains the second most common cause of maternal mortality and because a cure has proved elusive, early diagnosis and treatment remain the focus of antenatal care.
Predictors of later disease include family, obstetric, and medical histories and Doppler ultrasound of the uterine artery flow in midpregnancy. These may define very highrisk individuals, but serve only as broad screening methods. The raft of biochemical tests of placental and plasma proteins have been evaluated, but none has reached routine practice. The diagnosis is made from 2 imperfect measures of end organ involvement—hypertension and proteinuria, both reviewed in BMJ.
Cnossen and colleagues performed a meta-analysis that suggests that in low-risk populations, mean arterial blood pressure in the first half of pregnancy is a better predictor of later preeclampsia than diastolic and systolic readings or an increase in diastolic pressure. The article suggests that a mean arterial pressure of 90 mm Hg or higher prior to 20 weeks is the most telling measurement, with a sensitivity of 62% and a specificity of 82%. In high-risk groups, a diastolic pressure of greater than 75 mm Hg was the best predictor, but, again, its accuracy was modest.
An editorial by Walsh and Baxi states that the most commonly used cutoff point for the diagnosis of severe preeclampsia is a mean arterial pressure of 125 mm Hg.
The detection of proteinuria, usually in the face of rising blood pressure, confirms the diagnosis of preeclampsia and warrants admission. Significant proteinuria is accepted as 300 mg over 24 hours, but measuring is a tedious process with many causes of inaccurate results; it is not a practical screening mechanism. Although widely used, the urinary dipstick test is neither reliable nor quantitative, and its back-up with another spot test is reported by Côté and colleagues.
The protein to creatinine ratio is a spot test that can be quickly carried out by any laboratory that determines protein and creatinine concentrations in 24-hour specimens. The cutoff point of 30 mg/mmol appears helpful in that women with results below this threshold can be adjudged not to have significant proteinuria, which is important information to antenatal day clinics. It is not claimed to be an accurate means of quantifying proteinuria, which still remains an inpatient measure and a crucial determinant of the severity of preeclampsia. Perhaps the role for the urinary spot protein to creatinine ratio is to rule out significant proteinuria in outpatients who are urinary dipstick test-positive, thus saving the stress and cost of unnecessary hospital admission.