PAPP-A Unit Converter
PAPP-A Unit Converter
mIU/L and IU/L differ by a thousand, which is the whole hazard on this page. The second hazard is larger: first-trimester screening reports PAPP-A as a multiple of the median for gestation, and the raw concentration on its own means nothing.
PAPP-A converter
mIU/L ⇄ IU/LPAPP-A 2,500 mIU/L on a first-trimester screening report
Units, and why there is no mass unit
IU/L = mIU/mL = mIU/L ÷ 1000
MoM = measured PAPP-A ÷ the median PAPP-A for that exact gestational day
- mIU/L and µIU/mL
- numerically identical. A micro-international-unit per millilitre is a milli-international-unit per litre, because the thousandfold in the unit cancels the thousandfold in the volume
- IU/L and mIU/mL
- also identical to each other, and one thousand times larger than mIU/L. A PAPP-A of 2,500 mIU/L is 2.5 IU/L, and reading one report’s number against the other’s scale is a thousandfold error
- no ng/mL
- deliberately absent. The screening assays in routine use report international units against a common reference preparation, and no published calibration converts those units to a mass concentration. Papers that report PAPP-A in ng/mL use research assays with their own calibrators, so a factor taken from one of them would not apply to a screening report
- no molar unit
- PAPP-A circulates in pregnancy as a 500 kDa heterotetramer bound covalently to proMBP, and in other settings as the free dimer. Two different masses, so no single molar conversion exists
- MoM
- the multiple of the median — the measured concentration divided by the median for that gestational day in that laboratory’s own population, then adjusted for maternal weight, ethnicity, smoking, IVF conception and diabetes. Every published PAPP-A threshold is a MoM, never a concentration
Worked example
PAPP-A 2,500 mIU/L on a first-trimester screening report
2,500 mIU/L = 2,500 µIU/mL — identical units
2,500 ÷ 1000 = 2.500 IU/L = 2.500 mIU/mL
And that is as far as the arithmetic goes. Whether 2,500 mIU/L is low depends on the gestational day: the same figure is around the median at 12 weeks and well below it at 13, which is why the report carries a MoM
The MoM cannot be derived from this page, because it needs the laboratory's own gestation-specific median
The same PAPP-A in every unit a report uses
| mIU/L | µIU/mL | IU/L | mIU/mL |
|---|---|---|---|
| 250 | 250 | 0.250 | 0.250 |
| 1,000 | 1,000 | 1.000 | 1.000 |
| 2,500 | 2,500 | 2.500 | 2.500 |
| 5,000 | 5,000 | 5.000 | 5.000 |
| 10,000 | 10,000 | 10.000 | 10.000 |
What the first-trimester combined test actually uses
| Component | Detail |
|---|---|
| Window | Crown–rump length 45.0 to 84.0 mm, which is roughly 11 to 14 weeks |
| Markers | Maternal age, nuchal translucency, free beta-hCG and PAPP-A |
| How PAPP-A enters | As a MoM for the exact gestational day, adjusted for maternal weight, ethnicity, smoking, IVF conception and diabetes — never as a raw concentration |
| Reported as | A chance, not a marker value. The NHS programme cut-off is 1 in 150 at term, at or above which the result is higher-chance |
| Low PAPP-A and growth | Below 0.415 MoM, the 5th centile, RCOG Green-top Guideline 31 treats low PAPP-A as a major risk factor for a small-for-gestational-age neonate and a reason for increased growth surveillance |
A number that is not interpretable on its own
Pregnancy-associated plasma protein A is a zinc metalloproteinase that cleaves insulin-like growth factor binding protein 4, releasing bound IGF at the placental surface. It is produced by the syncytiotrophoblast, and its concentration in maternal serum climbs steeply through the first trimester — which is the reason a raw value cannot be read against a fixed range. The median at eleven weeks is not the median at thirteen, and a concentration that is unremarkable on one day is well below the median a week later.
Screening handles this by converting the measurement to a multiple of the median. The laboratory divides the measured concentration by the median for that exact gestational day in its own population, then adjusts for maternal weight, ethnicity, smoking, IVF conception and diabetes, and only the resulting MoM enters the risk calculation alongside nuchal translucency, free beta-hCG and maternal age. Every published PAPP-A threshold in the literature is expressed as a MoM for this reason. The MoM cannot be recovered from the concentration without the laboratory’s own median curve, so no converter — this one included — can produce it. If a report gives a concentration but no MoM, the concentration is not actionable and the laboratory should be asked for the MoM.
The unit hazard is genuine but simple. mIU/L and µIU/mL are the same number. IU/L and mIU/mL are the same as each other and a thousand times larger, so a PAPP-A of 2,500 mIU/L is 2.5 IU/L. A report read in the wrong pair is out by three orders of magnitude. There is no mass unit here on purpose: the assays used for screening are calibrated in international units against a common reference preparation, and no published factor converts those units to nanograms per millilitre. The cardiology literature, where PAPP-A has been studied as a plaque-instability marker, does report mass concentrations, but from research assays with their own calibrators and in a setting where PAPP-A circulates as the free dimer rather than bound to proMBP — a different molecule at a different mass. Carrying a mass column would have meant inventing a factor.
One threshold is worth knowing outside aneuploidy screening. A PAPP-A below 0.415 MoM, the fifth centile, is treated by RCOG Green-top Guideline 31 as a major risk factor for delivering a small-for-gestational-age neonate, and is a reason for increased ultrasound surveillance of fetal growth. That is a MoM threshold, it applies to the first-trimester sample, and it says nothing about aneuploidy.
Frequently asked questions
How do I convert PAPP-A from mIU/L to IU/L?
Divide by 1000. A PAPP-A of 2,500 mIU/L is 2.5 IU/L. mIU/mL is identical to IU/L, and µIU/mL is identical to mIU/L, so the four units on this page are really two, a thousandfold apart.
Why is there no ng/mL option for PAPP-A?
Because no published calibration converts the international units used by screening assays into a mass concentration. Research assays that report PAPP-A in ng/mL use their own calibrators and, outside pregnancy, measure a different form of the molecule. A conversion factor here would have been invented rather than sourced.
What is a normal PAPP-A level in pregnancy?
There is no fixed normal concentration, because PAPP-A rises steeply through the first trimester. Screening reports it as a multiple of the median for the exact gestational day, adjusted for maternal characteristics, and only that MoM is interpretable.
What does a low PAPP-A mean?
In aneuploidy screening it contributes to the calculated chance alongside nuchal translucency, free beta-hCG and maternal age; it is not a result on its own. Separately, a PAPP-A below 0.415 MoM is a major risk factor for a small-for-gestational-age neonate under RCOG Green-top Guideline 31 and a reason for extra growth surveillance.
Can I work out the MoM from the concentration?
Not from this page or any other converter. The MoM is the concentration divided by the median for that gestational day in the reporting laboratory’s own population, then adjusted for weight, ethnicity, smoking, IVF and diabetes. Without that median curve the MoM cannot be reconstructed.
Related calculators
References
- Royal College of Obstetricians and Gynaecologists. The Investigation and Management of the Small-for-Gestational-Age Fetus. Green-top Guideline No. 31.
- NHS Fetal Anomaly Screening Programme. Screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome. FASP handbook.
- Lambert-Messerlian G, Palomaki GE, Canick JA. Examination of the pregnancy-associated plasma protein-A assay on the Beckman Coulter Access platform: suitability for use in first trimester Down’s syndrome screening. J Med Screen. 2010;17(2):64–68.
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.