sFlt-1/PlGF Ratio Calculator
sFlt-1/PlGF Ratio Calculator
Calculate the angiogenic ratio used to rule out pre-eclampsia, on the platform your laboratory actually runs — because the same ratio is a rule-out on one assay and intermediate on another.
sFlt-1/PlGF Ratio
Angiogenic ratio, platform-specificsFlt-1 2,400 pg/mL and PlGF 60 pg/mL, before 34 weeks
Formula
- sFlt-1
- soluble Fms-like tyrosine kinase-1 — an anti-angiogenic protein released by the placenta
- PlGF
- placental growth factor — a pro-angiogenic protein that sFlt-1 sequesters
- the ratio
- dimensionless, so the pg/mL cancels — but the two assays behind it are not standardised to each other, which is why the thresholds are not transferable
- thresholds
- platform-specific AND gestation-specific; see the table below for all three platforms NICE DG49 lists
Worked example
sFlt-1 2,400 pg/mL and PlGF 60 pg/mL, before 34 weeks
2,400 ÷ 60 = 40.0
On the Roche Elecsys ratio this is intermediate: above the one-week rule-out of 38, at or below the rule-in of 85
On the DELFIA Xpress ratio the same number is a rule-out, because that platform’s rule-out is 50 or less
Same sample, same arithmetic, opposite clinical conclusions — and neither is an error. This is why the platform is an input on this page and not a footnote
Now change one thing: at 34 weeks or later on Elecsys, 40.0 is still intermediate, but the upper edge of that band has moved from 85 to 110 — because PlGF falls physiologically as pregnancy advances
Every threshold NICE DG49 publishes, by platform
| Platform | Gestation | Rules out | Rules in |
|---|---|---|---|
| Roche Elecsys ratio | 20 to 33+6 | 33 (diagnosis) / 38 or less (1 week) | Over 85 |
| Roche Elecsys ratio | 34+ | 38 or less (1 week) | Over 110 |
| DELFIA Xpress ratio | 20 to 33+6 | 50 or less | 70 or over |
| DELFIA Xpress ratio | 34+ | 50 or less | 90 or over |
| BRAHMS Kryptor PE ratio | Not split | Not published | More than 85 |
The ratio is dimensionless. The threshold is not.
Pre-eclampsia is a disorder of placental angiogenic balance. The placenta releases soluble Fms-like tyrosine kinase-1, which sequesters placental growth factor and vascular endothelial growth factor, and it is the resulting endothelial dysfunction that produces the hypertension and proteinuria that define the clinical syndrome. The ratio measures that imbalance directly, ahead of the clinical signs it eventually causes, and its greatest value is negative: a low ratio safely avoids admitting and monitoring a woman who is not going to develop the disease this week.
It is tempting to treat the ratio as platform-independent, because dividing pg/mL by pg/mL cancels the units and leaves a bare number. That reasoning is wrong, and it is the single most important thing on this page. The two immunoassays behind the number are not standardised to each other, so the ratio inherits both calibrations — and NICE DG49 accordingly lists three platforms with different figures. A ratio of 40 before 34 weeks is intermediate on the Roche Elecsys assay and a rule-out on the Revvity DELFIA Xpress assay. Same sample, same division, opposite conclusions, neither of them a mistake.
Two subtleties are worth knowing because they change what happens at exactly the threshold. First, the inequality direction is not uniform: DG49 prints the Elecsys rule-in as over 85, so a ratio of exactly 85 is intermediate, while the DELFIA Xpress rule-in is 70 or over, so exactly 70 is positive. Second, Elecsys carries two different rule-outs for different questions — 33 for diagnosis between 20 and 33+6 weeks, and 38 or less for ruling out disease over the following week, the PROGNOSIS figure. Pairing one platform’s rule-out with another’s rule-in, or a prediction threshold with a diagnostic one, produces a confident answer to a question nobody asked.
None of this makes the ratio a diagnosis. It is a risk-stratification tool used alongside blood pressure, proteinuria and clinical assessment, and a high ratio in a well woman with a normal blood pressure is a reason to watch closely, not a reason to deliver.
Frequently asked questions
Why does this calculator ask which assay platform was used?
Because the thresholds differ, by enough to change the decision. NICE DG49 gives the Roche Elecsys ratio a one-week rule-out of 38 or less and the Revvity DELFIA Xpress ratio a rule-out of 50 or less. A ratio of 45 is reassuring on one platform and not on the other. The ratio being dimensionless does not make it standardised.
What does a low sFlt-1/PlGF ratio mean?
On the Elecsys assay, a ratio of 38 or less rules out pre-eclampsia developing within the next week with about 99% negative predictive value, from the PROGNOSIS study. That is the test’s strongest property and the reason it is used at presentation.
Is the rule-in threshold the same at every gestation?
No, and it rises rather than falls. PlGF declines physiologically as pregnancy advances, so the same angiogenic imbalance produces a higher ratio later on. On Elecsys the rule-in moves from over 85 before 34 weeks to over 110 from 34 weeks; on DELFIA Xpress, from 70 or over to 90 or over. The rule-out does not move on either.
Why are there two different Elecsys rule-out figures, 33 and 38?
They answer different questions. DG49 lists 33 as the diagnostic rule-out for 20 to 33+6 weeks, and 38 or less as the threshold for ruling out pre-eclampsia developing within the next week across 24 to 36+6 weeks — the PROGNOSIS figure. This page bands on 38 and names 33 in the note, because the one-week question is the one asked at presentation.
Does a high ratio diagnose pre-eclampsia on its own?
No. It is a risk-stratification tool interpreted alongside blood pressure, proteinuria and the clinical picture, not a standalone diagnostic test.
The BRAHMS Kryptor option shows no rule-out. Why?
Because DG49 publishes only one figure for that platform — more than 85 suggests pre-eclampsia — with no separate rule-out and no gestation split. Borrowing 38 or 50 from another platform would be inventing a threshold, so the page says the published threshold has not been crossed and leaves it there.
Related calculators
References
- National Institute for Health and Care Excellence. PLGF-based testing to help diagnose suspected preterm pre-eclampsia. NICE Diagnostics Guidance DG49 (update of DG23), chapter 2: The diagnostic tests.
- Zeisler H et al. Predictive value of the sFlt-1:PlGF ratio in women with suspected preeclampsia. N Engl J Med. 2016;374(1):13–22.
- Stepan H et al. Clinical utility of sFlt-1 and PlGF in screening, prediction, diagnosis and monitoring of pre-eclampsia and fetal growth restriction. Ultrasound Obstet Gynecol. 2023;61(2):168–80.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/