CA 15-3 Unit Converter
CA 15-3 Unit Converter
U/mL and kU/L are the same number for CA 15-3. The conventional cut-off is about 30 U/mL, and the test is for monitoring known breast cancer — never for diagnosing it.
CA 15-3 converter
U/mL ⇄ kU/LCA 15-3 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
- arbitrary units
- defined by the assay calibrator against the DF3 and 115D8 antibodies, not by mass, so no molar conversion exists
- MUC1
- the measured antigen is a circulating fragment of the MUC1 mucin, a very large and variably glycosylated molecule with no single molecular mass
- CA 27.29
- a different assay against the same MUC1 antigen, with its own cut-off; results from the two are not interchangeable
Worked example
CA 15-3 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 30 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 |
What CA 15-3 is and is not for
| Use | Verdict |
|---|---|
| Screening healthy people for breast cancer | No — it is normal in most early-stage disease |
| Diagnosing a breast lump | No — imaging and biopsy decide this, whatever the marker shows |
| Monitoring response in metastatic breast cancer | Yes, alongside imaging and clinical assessment |
| Serial surveillance for recurrence after treatment | Sometimes, in conjunction with clinical information; a confirmed rising trend is what counts |
| Comparing a CA 15-3 with a CA 27.29 | No — different assays against the same antigen, not interchangeable |
A monitoring marker, and only that
CA 15-3 is reported in U/mL, kU/L or kIU/L, and those are the same number, since a unit per millilitre is a kilounit per litre. U/L alone differs, being a thousand times larger. There is no molar unit and there cannot be one: the assay measures a circulating fragment of MUC1, a very large and variably glycosylated mucin, in arbitrary units defined by its own calibrator. No molecular mass applies, so any pmol/L figure offered for CA 15-3 would be fabricated.
The conventional upper limit is about 30 U/mL, although it is assay-specific and the report’s own figure should be used. A related test, CA 27.29, measures the same MUC1 antigen with different antibodies and a different cut-off, and the two cannot be compared with each other. Nor can CA 15-3 results from different manufacturers’ platforms, which is why serial monitoring is only interpretable when the laboratory and the assay stay the same.
The role of this test is monitoring, not diagnosis. CA 15-3 is normal in the great majority of early-stage breast cancer, so it cannot detect disease and is not a screening test; a lump, a skin or nipple change, or an abnormal mammogram is investigated by imaging and biopsy whatever the marker shows. Where it earns its place is in metastatic disease, where serial values alongside imaging and clinical assessment help judge response to treatment, and in surveillance after treatment, where a confirmed rising trend can prompt earlier restaging.
Two traps recur. The first is the single raised value: benign breast disease, cirrhosis and chronic hepatitis, sarcoidosis, tuberculosis and other chronic inflammatory conditions all raise CA 15-3, so one result above the cut-off means little on its own and needs confirmation. The second is the spurious early rise, a transient increase in the first weeks of chemotherapy or endocrine therapy that reflects tumour cell lysis rather than progression and settles without any change of treatment. Neither should be acted on without the treating team.
Frequently asked questions
Is U/mL the same as kU/L for CA 15-3?
Yes, and kIU/L too, because a unit per millilitre is a kilounit per litre. U/L is a thousand times larger, so 18 U/mL is 18 kU/L and 18,000 U/L. There is no molar unit for CA 15-3.
What is a normal CA 15-3?
Conventionally below about 30 U/mL, though the exact limit is assay-specific and the figure on the report should be used. A normal result does not exclude breast cancer — the marker is normal in most early-stage disease.
Can CA 15-3 diagnose breast cancer?
No. It is normal in the great majority of early-stage breast cancer and is raised by several benign conditions, so it is useless as a screening or diagnostic test. A lump or an abnormal mammogram is assessed by imaging and biopsy regardless of the marker.
Why did my CA 15-3 rise after starting chemotherapy?
A transient rise in the first weeks of chemotherapy or endocrine therapy is well described and is thought to reflect tumour cell lysis rather than progression. It typically settles, which is why an early increase is not acted on without repeat testing and imaging.
Is CA 15-3 the same as CA 27.29?
They measure the same MUC1 antigen but with different antibodies, different calibration and different cut-offs, so the numbers are not interchangeable. A patient followed on one should stay on it; switching assays can create an apparent trend that is not real.
Related calculators
References
- Duffy MJ, Evoy D, McDermott EW. CA 15-3: uses and limitation as a biomarker for breast cancer. Clin Chim Acta. 2010;411(23-24):1869–1874.
- Harris L, Fritsche H, Mennel R, et al. American Society of Clinical Oncology 2007 update of recommendations for the use of tumor markers in breast cancer. J Clin Oncol. 2007;25(33):5287–5312.
- Mayo Clinic Laboratories. Cancer Antigen 15-3 (CA 15-3), Serum: reference value <30 U/mL; not useful as a cancer screening test.
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.