sFlt-1/PlGF Ratio Calculator
sFlt-1/PlGF Ratio Calculator
Calculate the angiogenic ratio used to rule out pre-eclampsia within one week, and see how the rule-in threshold changes with gestation.
sFlt-1/PlGF Ratio
Angiogenic ratiosFlt-1 2,400 pg/mL, PlGF 60 pg/mL
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
- ≤ 38
- rules out pre-eclampsia within one week with about 99% certainty (PROGNOSIS study)
- rule-in threshold
- gestation-dependent — above 85 before 34 weeks, above 110 from 34 weeks
Worked example
sFlt-1 2,400 pg/mL, PlGF 60 pg/mL
2,400 ÷ 60 = 40.0
Rule-in thresholds by gestation
| Gestation | Threshold consistent with pre-eclampsia |
|---|---|
| Before 34 weeks | > 85 |
| From 34 weeks | > 110 |
An angiogenic imbalance, measured directly
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.
Its greatest value is negative predictive: a ratio of 38 or below rules out pre-eclampsia within the following week with about 99% certainty, a finding from the PROGNOSIS study, and this is what safely avoids unnecessary admission and monitoring for women presenting with suspected disease. Rule-in thresholds are less clean and differ by gestation — above 85 before 34 weeks and above 110 from 34 weeks — reflecting that PlGF itself falls physiologically as pregnancy advances.
The ratio is a risk-stratification tool used alongside blood pressure and proteinuria, not a diagnosis in its own right, and it does not replace clinical assessment. Cut-offs are also assay-specific — the values validated for one immunoassay platform do not transfer directly to another — so confirm which thresholds apply to the assay used by the reporting laboratory before acting on a borderline result.
Frequently asked questions
What does a low sFlt-1/PlGF ratio mean?
A ratio of 38 or below has about 99% negative predictive value for pre-eclampsia developing within the next week, from the PROGNOSIS study. It is most useful for safely ruling the diagnosis out.
Is the rule-in threshold the same at every gestation?
No. It is higher later in pregnancy because PlGF falls physiologically as gestation advances — above 85 is used before 34 weeks and above 110 from 34 weeks with the assay studied in PROGNOSIS.
Does a high ratio diagnose pre-eclampsia on its own?
No. It is a risk-stratification tool used alongside blood pressure and proteinuria, not a standalone diagnostic test, and results should be interpreted with the full clinical picture.
Can I use these thresholds with any laboratory's assay?
Not necessarily. Cut-offs are assay-specific. Confirm the thresholds validated for the immunoassay platform used by the reporting laboratory before applying them to a borderline result.
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References
- 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.
- National Institute for Health and Care Excellence. PlGF-based testing to help diagnose suspected pre-eclampsia. NICE Diagnostics Guidance DG23.