CA 125 Unit Converter
CA 125 Unit Converter
U/mL and kU/L are the same number, so the work on this page is the interpretation: the 35 U/mL convention, the long list of benign causes, and why CA 125 is not a screening test.
CA 125 converter
U/mL ⇄ kU/LCA 125 18 U/mL
Formula and conversion factors
kIU/L = U/mL
U/L = U/mL × 1000
- U/mL = kU/L
- a unit per millilitre is a kilounit per litre, so the two are the same number and no conversion is required
- arbitrary units
- the unit is defined by the assay's own calibrator against the OC 125 antibody, not by a mass or a molar quantity
- no molar unit
- CA 125 is an epitope on the very large, variably glycosylated MUC16 mucin; there is no single molecular mass, so no pmol/L conversion exists
- assay dependence
- second-generation CA 125 II assays differ between manufacturers by more than the conversion arithmetic ever would; follow a patient on one platform
Worked example
CA 125 18 U/mL
18 U/mL = 18 kU/L = 18 kIU/L — all the same number
18 × 1000 = 18,000 U/L
Below the conventional cut-off of 35 U/mL
The four units
| Unit | Relationship | Example |
|---|---|---|
| U/mL | The usual reporting convention | 18 |
| kU/L | Identical to U/mL | 18 |
| kIU/L | Identical to U/mL | 18 |
| U/L | U/mL × 1000 | 18,000 |
Raised CA 125 without ovarian cancer
| Cause | Note |
|---|---|
| Menstruation | Can raise CA 125 up to threefold — avoid sampling during a period |
| Endometriosis, fibroids, ovarian cysts | Common in premenopausal women and a frequent cause of a modest rise |
| Pregnancy, particularly the first trimester, and the postpartum period | Physiological elevation |
| Pelvic inflammatory disease | Elevation tracking the acute episode |
| Cirrhosis with ascites, heart failure, any pleural or peritoneal irritation | CA 125 is made by mesothelium, so serosal irritation raises it whatever the cause |
| Pancreatitis, recent abdominal or pelvic surgery, endometrial and other cancers | Non-specific elevation |
An arbitrary unit and a conventional cut-off
CA 125 is reported in U/mL, kU/L or kIU/L, and those three are the same number: a unit per millilitre is a kilounit per litre. Only U/L differs, being a thousand times larger. There is no molar column and there never can be. The unit is arbitrary, defined by each assay’s calibrator against the OC 125 antibody, and the target is an epitope on MUC16, a very large and variably glycosylated mucin with no single molecular mass. Anyone offering a pmol/L figure for CA 125 has invented it.
The familiar 35 U/mL cut-off is a statistical convention rather than a biological boundary. It was chosen because roughly 99 per cent of apparently healthy people fall below it, and it carries no guarantee about the individual sitting either side. Some laboratories apply a lower threshold after the menopause, where benign gynaecological causes are less common and the same number carries more weight.
The list of benign causes is long, and it is the single most important thing to know about this test. Menstruation alone can treble the result, so timing the sample matters. Endometriosis, fibroids, ovarian cysts, pelvic inflammatory disease, pregnancy and the postpartum period all raise it. So does anything that irritates a serosal surface, because CA 125 is produced by mesothelium: cirrhosis with ascites, heart failure, pleural effusion, pancreatitis and recent abdominal surgery. A raised CA 125 in a premenopausal woman is more often benign than not.
CA 125 is also not a screening test in the general population. Large randomised trials of screening with CA 125 and transvaginal ultrasound have not shown a reduction in ovarian cancer mortality, and roughly half of early-stage cancers give a result below 35 U/mL, with mucinous tumours frequently never raising it at all. Where the question is a suspicious pelvic mass, CA 125 is combined with imaging and menopausal status in a risk of malignancy index or the ROMA score. A number on its own, high or low, does not settle anything and needs a clinician to interpret it.
Frequently asked questions
Is U/mL the same as kU/L for CA 125?
Yes, and kIU/L is the same number too, because a unit per millilitre is a kilounit per litre. Only U/L differs: it is a thousand times larger, so 18 U/mL is 18 kU/L and 18,000 U/L.
Why is there no pmol/L for CA 125?
The unit is arbitrary, defined by each assay’s own calibrator rather than by mass, and the target is an epitope on MUC16, a very large and variably glycosylated mucin with no single molecular mass. There is nothing to divide by, so no molar conversion exists.
What raises CA 125 apart from ovarian cancer?
Menstruation, which can treble it, endometriosis, fibroids, ovarian cysts, pelvic inflammatory disease, pregnancy and the postpartum period. Also anything irritating a serosal surface — cirrhosis with ascites, heart failure, pleural effusion, pancreatitis and recent abdominal surgery.
Can CA 125 be used to screen for ovarian cancer?
No. Large randomised trials of screening with CA 125 and transvaginal ultrasound have not shown a reduction in ovarian cancer mortality, and the false-positive rate leads to unnecessary surgery. It is used in women with symptoms or a known mass, alongside imaging.
Does a normal CA 125 rule out ovarian cancer?
No. Roughly half of early-stage ovarian cancers give a result below 35 U/mL, and mucinous tumours often never raise it. Persistent bloating, early satiety, pelvic pain or urinary urgency should be assessed clinically whatever the CA 125 shows.
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References
- Charkhchi P, Cybulski C, Gronwald J, et al. CA125 and ovarian cancer: a comprehensive review. Cancers (Basel). 2020;12(12):3730.
- National Institute for Health and Care Excellence. Ovarian cancer: recognition and initial management. NICE clinical guideline CG122. London: NICE; 2011.
- Zhang M, Cheng S, Jin Y, et al. Cancer antigen 125. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. NBK562245.