sFlt-1 Unit Converter
sFlt-1 Unit Converter
Convert soluble fms-like tyrosine kinase-1 between pg/mL, ng/L, ng/mL and µg/L — the first two are the same number. As with its partner PlGF, there is no reference interval here on purpose: the thresholds in use belong to named assay platforms, and it is the ratio of the two, not either concentration, that the trials tested.
sFlt-1 converter
Mass units onlysFlt-1 2,400 pg/mL at 30 weeks, with a PlGF of 60 pg/mL on the same sample
The ladder, and why no molar unit appears
1 ng/mL = 1 µg/L = 1,000 pg/mL
no pmol/L is offered, and none should be
- pg/mL = ng/L
- a picogram in a millilitre is a nanogram in a litre; the thousandfold in the prefix cancels the thousandfold in the volume. Assay inserts and reports print pg/mL, and sFlt-1 concentrations in pregnancy run in the hundreds to tens of thousands on that scale
- ng/mL = µg/L
- both are the pg/mL figure divided by a thousand, and identical to each other. An sFlt-1 of 2,400 pg/mL appears as 2.4 on this rung — a number that looks like a plausible ratio rather than a concentration, which is the misreading worth guarding against
- no molar unit
- sFlt-1 is a circulating splice variant of VEGF receptor-1 lacking the transmembrane and kinase domains, around 110 kDa, glycosylated and measured against an assay calibrator. It has no single grams per mole, so a pmol/L figure would be an assumption dressed as arithmetic
- what the units cannot reconcile
- the platform. The ratio thresholds in routine use — 38 for short-term rule-out, and the 85 before 34 weeks and 110 after — are Roche Elecsys figures. The PerkinElmer DELFIA Xpress equivalents in the same national guidance are 50, 70 and 90. Both sets are correct on their own assay and neither transfers
Worked example
sFlt-1 2,400 pg/mL at 30 weeks, with a PlGF of 60 pg/mL on the same sample
2,400 pg/mL is 2,400 ng/L — the same number
2,400 ÷ 1,000 = 2.400 ng/mL, which is also 2.400 µg/L
No verdict follows from the concentration alone, and this page offers none: there is no sFlt-1 threshold in guidance that is not part of a ratio
With the paired PlGF of 60 pg/mL the ratio is 2,400 ÷ 60 = 40, which on the Roche assay is above the short-term rule-out of 38 and on the PerkinElmer assay is below its rule-out of 50 — the same sample, opposite conclusions, no arithmetic error
Take both numbers and the platform name to the sFlt-1/PlGF ratio calculator
The ratio thresholds, and the platform each one belongs to
| Test and platform | Rule out | Rule in |
|---|---|---|
| Elecsys sFlt‑1/PLGF ratio (Roche), 20 to 33+6 weeks | 33 | 85 |
| Elecsys sFlt‑1/PLGF ratio (Roche), 34 weeks to birth | 33 | 110 |
| Elecsys sFlt‑1/PLGF ratio (Roche), short-term prediction, 24 to 36+6 weeks | "≤38" rules out pre-eclampsia for one week | ">38" |
| DELFIA Xpress sFlt‑1/PLGF ratio (PerkinElmer), 20 to 33+6 weeks | "50 or less" for short-term rule out | ≥70 |
| DELFIA Xpress sFlt‑1/PLGF ratio (PerkinElmer), 34 weeks and later | "50 or less" for short-term rule out | ≥90 |
| BRAHMS sFlt‑1 Kryptor / BRAHMS PLGF plus Kryptor PE ratio (Thermo Fisher) | — | "A ratio of more than 85 suggests pre-eclampsia" |
What sFlt-1 is, and why it is not read alone
| Property | Consequence for the number |
|---|---|
| A soluble splice variant of VEGF receptor-1, around 110 kDa, lacking the transmembrane and kinase domains | No fixed molar mass, so no molar unit is offered — the ladder is mass-only by necessity |
| Acts by binding and sequestering circulating VEGF and PlGF | It causes the fall in free PlGF that the paired assay detects, which is why the two move in opposite directions and the ratio amplifies both |
| Rises before the clinical onset of pre-eclampsia | A prognostic and rule-out signal rather than a diagnostic one |
| Quantified against each manufacturer’s own calibrator | Concentrations are platform-specific, and so are all the ratio thresholds derived from them |
| No guidance publishes an sFlt-1 threshold outside a ratio | There is nothing to print as a reference interval for this analyte on its own |
The antagonist half of the pair, and why it is never read alone
The units first, because they are the part this page can settle. A picogram per millilitre and a nanogram per litre are the same concentration — the prefix and the volume each change a thousandfold and cancel — and a nanogram per millilitre and a microgram per litre are likewise the same as each other and are both the pg/mL figure divided by a thousand. Assays and reports use pg/mL, on which scale sFlt-1 in pregnancy runs from the hundreds into the tens of thousands. The risk the ladder guards against is a result printed on the lower rung: an sFlt-1 of 2,400 pg/mL appears as 2.4, which looks less like a concentration than like a ratio, and is the one way this number gets badly misread.
No molar unit is offered. sFlt-1 is a circulating splice variant of vascular endothelial growth factor receptor-1, around a hundred and ten kilodaltons, lacking the transmembrane and kinase domains, glycosylated and quantified against a manufacturer’s protein calibrator. There is no single grams per mole to divide by, so a pmol/L figure would be an assumption presented as a conversion, and the build for this site refuses one rather than relying on an author to remember.
What sFlt-1 does explains why it is never read on its own. It is the antagonist: circulating sFlt-1 binds and sequesters vascular endothelial growth factor and placental growth factor, which is precisely why free PlGF falls as sFlt-1 rises. The two therefore move in opposite directions in the same process, and their ratio moves further and earlier than either component — which is the whole reason the ratio, and not this concentration, is what has been trialled and what guidance recommends. The PROGNOSIS study of 550 women with suspected pre-eclampsia found that an sFlt-1:PlGF ratio of 38 or lower had a negative predictive value of 99.3 per cent for pre-eclampsia in the following week, and that a ratio above 38 had a positive predictive value of 36.7 per cent over four weeks. Strong rule-out, weak rule-in: that is the shape of the test, and it is worth knowing before ordering it.
The reason this page carries no reference interval is that there is no sFlt-1 threshold in guidance to carry — and that the ratio thresholds which do exist belong to particular analysers rather than to the measurand. The figure most clinicians know, a ratio of 38 for short-term rule-out, is a Roche Elecsys number. The PerkinElmer DELFIA Xpress equivalent, in the same national diagnostics guidance, is 50. The two-sided pair of 85 before 34 weeks and 110 from 34 weeks is also Roche’s; PerkinElmer’s are 70 and 90, and the Thermo Fisher BRAHMS Kryptor ratio uses a flat 85 with no gestational split. A ratio of 40 is therefore above the rule-out on one recommended platform and below it on another, with nobody having made a mistake. When an angiogenic threshold is quoted without the assay being named, that ambiguity travels with it, and the practical discipline is to record the platform beside the result.
One further limit belongs on this page. A reassuring angiogenic result does not define or undefine pre-eclampsia, which remains a clinical and laboratory diagnosis of hypertension with proteinuria or maternal organ dysfunction. A low ratio lowers the probability of the syndrome developing over the next week; it does not overrule a rising blood pressure, a rising protein-creatinine ratio, falling platelets or deranged transaminases. Convert the number here, take it with its paired result to the sFlt-1/PlGF ratio calculator, and read the syndrome itself on the pre-eclampsia laboratory criteria interpreter and the urine protein to creatinine ratio calculator. The companion converter for the other half of the pair is the PlGF converter.
Frequently asked questions
Is pg/mL the same as ng/L for sFlt-1?
Yes, exactly. A picogram per millilitre and a nanogram per litre are the same concentration, because the thousandfold change in the prefix cancels the thousandfold change in the volume. Nanograms per millilitre and micrograms per litre are identical to each other and are both the pg/mL figure divided by a thousand, so an sFlt-1 of 2,400 pg/mL is 2.400 ng/mL. Assays report in pg/mL.
Why is there no pmol/L option for sFlt-1?
Because there is no fixed molar mass to convert against. sFlt-1 is a soluble splice variant of VEGF receptor-1, around 110 kDa, glycosylated and measured against each manufacturer’s protein calibrator rather than against a defined molecular weight. A pmol/L figure would require assuming a mass and would present that assumption as arithmetic, so the ladder on this page is mass-only.
What is a normal sFlt-1 level?
No guidance publishes one, which is why this page prints none. Every threshold in clinical use is a ratio to PlGF rather than an sFlt-1 concentration, and those ratio thresholds are themselves platform-specific. An isolated sFlt-1 has no interpretation attached to it; it becomes interpretable only alongside a PlGF measured on the same sample by the same assay.
Where does the sFlt-1/PlGF ratio of 38 come from, and does it apply to every assay?
It comes from the PROGNOSIS study and is a Roche Elecsys figure: in 550 women with suspected pre-eclampsia, a ratio of 38 or lower had a negative predictive value of 99.3% for pre-eclampsia in the following week. It does not transfer. In the same national guidance the PerkinElmer DELFIA Xpress short-term rule-out is 50, its rule-in thresholds are 70 before 34 weeks and 90 after, against Roche’s 85 and 110, and the Thermo Fisher BRAHMS Kryptor ratio uses a flat 85. A ratio of 40 is above the rule-out on one platform and below it on another.
Why is sFlt-1 always measured with PlGF?
Because sFlt-1 is the antagonist that causes the PlGF signal. Circulating sFlt-1 binds and sequesters VEGF and placental growth factor, so free PlGF falls as sFlt-1 rises, and the ratio of the two moves further and earlier than either component. That amplification is what gives the ratio its rule-out performance, and it is the ratio rather than either concentration that the trials evaluated and that guidance recommends.
Does a low sFlt-1/PlGF ratio exclude pre-eclampsia?
It substantially lowers the probability over the following week and does not exclude the diagnosis. Pre-eclampsia is defined by hypertension with proteinuria or maternal organ dysfunction, and an angiogenic result does not overrule clinical deterioration, a rising urine protein-creatinine ratio, falling platelets or rising transaminases. The biomarkers are rule-out tools used inside a clinical assessment, and the positive predictive value of a high ratio is only around 37% over four weeks, so they are much weaker at ruling in.
Related calculators
References
- National Institute for Health and Care Excellence. PLGF-based testing to help diagnose suspected preterm pre-eclampsia. Diagnostics guidance DG49. Elecsys sFlt‑1/PLGF ratio (Roche): rule out 33, rule in 85 at 20–33+6 weeks and 110 from 34 weeks, short-term rule out "≤38". DELFIA Xpress sFlt‑1/PLGF ratio (PerkinElmer): rule in ≥70 at 20–33+6 weeks and ≥90 from 34 weeks, short-term rule out "50 or less". BRAHMS sFlt‑1 Kryptor / BRAHMS PLGF plus Kryptor PE ratio (Thermo Fisher): "A ratio of more than 85 suggests pre-eclampsia". "The tests should be used according to their indications for use."
- Zeisler H, Llurba E, Chantraine F, et al. Predictive value of the sFlt-1:PlGF ratio in women with suspected preeclampsia. N Engl J Med. 2016;374(1):13–22. Validation cohort of 550 women; "an sFlt-1:PlGF ratio of 38 or lower had a negative predictive value (i.e., no preeclampsia in the subsequent week) of 99.3% (95% confidence interval [CI], 97.9 to 99.9)"; positive predictive value above 38 of 36.7%; "An sFlt-1:PlGF ratio of 38 or lower can be used to predict the short-term absence of preeclampsia".
- CalcEngines _factors.json, analyte
sflt1: "a ~110 kDa circulating splice variant lacking the transmembrane and kinase domains, reported as a mass concentration; pg/mL and ng/L are identical. As with PlGF, the published ratio thresholds (38, and the 85/110 two-sided pair) belong to a named platform, not to the measurand."
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.