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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Obstetrics

What is the sFlt-1/PlGF Ratio Test? | Inciteful Med

At a Glance

The sFlt-1/PlGF ratio is a blood test that predicts if a pregnant woman will develop severe preeclampsia. A low ratio (38 or less) means the condition is unlikely to worsen soon, while a high ratio indicates placental stress and that early delivery may be necessary.

When your doctor mentions a new blood test to predict if you will develop severe preeclampsia, they are talking about the sFlt-1/PlGF ratio. This is not a routine screening test given to all pregnant women; it is a specific tool used when a mother is already showing signs like high blood pressure. The test itself is a simple maternal blood draw from your arm—it is perfectly safe and poses absolutely zero risk to your baby.

This test measures the balance of two specific proteins released by the placenta into your bloodstream [1]. Rather than just diagnosing preeclampsia, this test acts as a highly accurate forecasting tool [2]. It helps your care team predict with great confidence whether your condition is likely to remain stable or become severe over the next one to four weeks [3][4].

What are sFlt-1 and PlGF?

To understand this test, it helps to look at how a healthy placenta functions. The placenta relies on a rich network of blood vessels to deliver oxygen and nutrients to your baby.

  • PlGF (Placental Growth Factor): This is a “pro-angiogenic” protein, meaning it promotes the growth and health of blood vessels [1]. In a healthy pregnancy, your placenta produces plenty of PlGF to keep blood flowing smoothly [5].
  • sFlt-1 (soluble fms-like tyrosine kinase-1): Think of this protein as a sponge that soaks up PlGF. When the placenta is stressed or not getting enough oxygen, it releases large amounts of sFlt-1 into the mother’s bloodstream [1][6]. This excess sFlt-1 binds to the helpful PlGF proteins, preventing them from doing their job [7]. This disruption damages the mother’s blood vessels (endothelial dysfunction), leading to high blood pressure and other symptoms of preeclampsia [8].

Understanding Your Ratio Numbers

By dividing the amount of sFlt-1 by the amount of PlGF in your blood, doctors get a ratio that reveals how stressed your placenta is. The higher the number, the higher the risk.

  • A Low Ratio (38 or lower): If your ratio is 38 or less, there is a very high probability that your preeclampsia will not worsen or require delivery within the next week [3][9]. For many women, this result provides immense peace of mind and often means you can safely monitor your pregnancy from home rather than being admitted to the hospital [10][11].
  • An Intermediate Ratio (39 to 84, or up to 109): If your score falls in this middle zone, it means you are at a higher risk of developing severe preeclampsia within the next four weeks, but you are generally not in immediate danger [12]. Your doctor will likely recommend closer monitoring and may repeat the blood test in a few days or a week to see if the numbers are climbing.
  • A High Ratio (85 or higher for early pregnancy; 110 or higher for late pregnancy): The threshold for a “high” ratio depends on how far along you are. If you are before 34 weeks of pregnancy, a score of 85 or higher is considered high. If you are past 34 weeks, the cutoff is 110 [13]. A high number indicates that the placenta is struggling and the disease is likely to progress. Very high ratios (such as those over 655) are associated with a rapid worsening of severe preeclampsia and a shorter timeline before it becomes safer to deliver the baby [12].

How This Test Changes Your Care Plan

Before this test was available, doctors had to rely heavily on symptoms, blood pressure readings, and urine protein levels, which can sometimes fluctuate or be misleading [14]. The sFlt-1/PlGF ratio provides a direct look at the root cause of the disease—the placenta.

However, this test is an addition to your care, not a replacement. Your doctor will use your ratio alongside your blood pressure readings, physical symptoms, and ultrasound results to make decisions [15][16]. If your ratio is low, your doctor may recommend safely waiting to give your baby more time to develop. If the ratio climbs, your care team will know it is time to shift gears, increase monitoring, or prepare for delivery [17].

Common questions in this guide

What is the sFlt-1/PlGF ratio blood test used for?
This blood test helps predict whether a pregnant woman showing signs of preeclampsia, like high blood pressure, will develop severe preeclampsia in the next one to four weeks. It helps doctors decide if it is safe to wait or if early delivery is needed.
What does a low sFlt-1/PlGF ratio mean?
A ratio of 38 or lower means there is a very high probability your preeclampsia will not worsen or require delivery within the next week. This often allows you to safely monitor your pregnancy from home.
What causes a high sFlt-1/PlGF ratio?
When the placenta is stressed or lacks oxygen, it releases excess sFlt-1. This protein acts like a sponge, soaking up the helpful PlGF proteins that keep blood vessels healthy, which leads to blood vessel damage and high blood pressure.
What is considered a high sFlt-1/PlGF ratio?
If you are before 34 weeks of pregnancy, a score of 85 or higher is considered high. If you are past 34 weeks, a ratio of 110 or higher indicates your placenta is struggling and the disease is likely to progress.
How does the sFlt-1/PlGF blood test change my pregnancy care plan?
If your ratio is climbing, your doctor will increase monitoring or prepare for delivery. If your ratio is low, your doctor may recommend safely waiting to give your baby more time to develop in the womb.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my exact sFlt-1/PlGF ratio number, and what category of risk does it place me in?
  2. 2.Based on my current gestational age, what cutoff number are we looking at to determine if my condition is severe?
  3. 3.If my ratio is low today, how frequently will we repeat the test over the next few weeks?
  4. 4.Is this specific blood test performed at our hospital's lab, and how quickly will we get the results back?
  5. 5.How does my current ratio affect our timeline for when it will be safest to deliver the baby?

Questions For You

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References

References (17)
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    Two decades of advances in preeclampsia research: molecular mechanisms and translational studies.

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    Circulating Angiogenic Factor Levels in Hypertensive Disorders of Pregnancy.

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    Soluble fms-like tyrosine kinase-1 to placental growth factor ratio: ruling out pre-eclampsia for up to 4 weeks and value of retesting.

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    Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology 2019; (53(3)):367-375 doi:10.1002/uog.19178.

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    The role of angiogenic factors in the management of preeclampsia.

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    Maternal serum soluble fms-like tyrosine kinase-1 at 12, 22, 32 and 36 weeks' gestation in screening for pre-eclampsia.

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This page explains the sFlt-1/PlGF ratio test for educational purposes only and does not replace professional medical advice. Always discuss your symptoms and test results with your obstetrician.

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