Acid Phosphatase Unit Converter
Acid Phosphatase Unit Converter
Convert acid phosphatase activity between U/L, IU/L, µkat/L and nkat/L. Total and prostatic acid phosphatase are different measurands with different intervals, and a modern prostatic acid phosphatase reported in ng/mL is an immunoassay that does not convert to U/L at all.
Acid phosphatase converter
Activity unitsTotal acid phosphatase 3.2 U/L
Formula and conversion factor
U/L = µkat/L × 60
nkat/L = U/L × 16.667
ng/mL does not convert — see below
- U
- 1 unit = 1 µmol of substrate converted per minute under the assay's own conditions
- kat
- 1 katal = 1 mol of substrate converted per second — the SI unit of catalytic activity
- ÷ 60
- the whole of the conversion. Per-minute against per-second, nothing else; no molecular weight is involved because activity is not a mass
- IU/L
- numerically identical to U/L. The I adds nothing here
- what does not convert
- a result reported in ng/mL or µg/L is a mass concentration measured by immunoassay, and there is no factor between it and U/L. The two tests count different things
Worked example
Total acid phosphatase 3.2 U/L
3.2 ÷ 60 = 0.053 µkat/L
= 53 nkat/L = 3.2 IU/L
Within the 0–5.1 U/L total acid phosphatase interval quoted for this reagent at 37 °C
That verdict would be wrong if the 3.2 were a prostatic fraction, whose interval on the same insert is 0–1 U/L
Three enzymes, one name
| What is measured | Where it comes from | What it is used for now | Units you will see |
|---|---|---|---|
| Total acid phosphatase | Prostate, platelets, red cells, osteoclasts, liver, spleen, kidney | Very little. Non-specific by construction, and raised by haemolysis or delayed separation | U/L, µkat/L |
| Prostatic acid phosphatase by tartrate inhibition | The tartrate-inhibitable fraction of total activity | Historical. Superseded by immunoassay where PAP is measured at all | U/L, µkat/L |
| Prostatic acid phosphatase by immunoassay | Prostatic epithelium — the protein, catalytically active or not | The modern PAP test. Largely displaced by PSA | ng/mL, µg/L |
| Tartrate-resistant acid phosphatase 5b (TRACP 5b) | Osteoclasts only | A bone resorption marker, and a different question entirely | U/L on its own dedicated assay |
The same reagent at two temperatures
| 30 °C | 37 °C | |
|---|---|---|
| Total acid phosphatase | 0 – 4.5 U/L | 0 – 5.1 U/L |
| Prostatic fraction | 0 – 0.8 U/L | 0 – 1.0 U/L |
A marker that was replaced, and the two things it is still for
Acid phosphatase is not one enzyme. It is a family of phosphatases that work at acid pH, present in prostate, platelets, red cells, osteoclasts, liver, spleen and kidney. Measuring total activity therefore measures all of them at once, which is why the total is close to useless as a prostate test and why a haemolysed sample, or one left on the clot, reads high for reasons that have nothing to do with the patient. The prostatic fraction was separated from the rest by the observation that prostatic acid phosphatase is inhibited by tartrate and the osteoclast enzyme is not — assay the sample twice, with and without tartrate, and the difference is the prostatic contribution.
That fraction was the first serum marker of metastatic prostate cancer and it is no longer used for the job. Mayo Clinic Laboratories states that prostatic acid phosphatase is no longer used to screen for or stage prostate cancer, that it provides little additional information beyond prostate-specific antigen, and that PSA provides more information and should also be used wherever PAP is being considered. Read a raised PAP in that light: it is not an alternative to PSA, and the page you want for a current prostate workup is the PSA one.
There is a further trap, and it is the reason this page leads with it. A prostatic acid phosphatase result issued today is almost always an immunoassay reported in ng/mL — Mayo quotes 2.1 ng/mL or less by chemiluminescent assay, Labcorp 0 to 3.5 ng/mL by immunochemiluminometric assay. That is a mass concentration of protein. The activity result this converter handles is a rate of catalysis. The immunoassay counts enzyme molecules whether or not they are working, the activity assay counts work done by however many molecules are present, and no fixed number converts one into the other. If your report says ng/mL, nothing on this page applies to it.
Two uses of acid phosphatase are not historical. Prostatic acid phosphatase is the antigen in sipuleucel-T, whose active component PA2024 is a PAP–granulocyte-macrophage colony-stimulating factor fusion protein used in metastatic castration-resistant prostate cancer — the marker outlived its own diagnostic use by becoming a target. And tartrate-resistant acid phosphatase isoform 5b is secreted only by osteoclasts, where isoform 5a comes from macrophages and dendritic cells, which makes TRACP 5b a marker of osteoclast number and a bone resorption test rather than a tumour marker. The cytochemical tartrate-resistant stain was also the classical bench test for hairy cell leukaemia, though that diagnosis now rests on immunophenotype and the BRAF V600E mutation. None of the three is the same test, and none of them is what a total acid phosphatase measures.
An enzyme activity is defined by the assay that measured it — its substrate, its buffer, its temperature. A reference interval belongs to that method and does not travel to another one, which is why the interval on your own report is the one to read the result against.
Frequently asked questions
How do I convert acid phosphatase from U/L to µkat/L?
Divide by 60. A total acid phosphatase of 3.2 U/L is 0.053 µkat/L, or 53 nkat/L. The factor is the katal definition — 1 unit is 1 µmol per minute and 1 katal is 1 mol per second — so it is exact and the same for every enzyme.
Can I convert a prostatic acid phosphatase in ng/mL to U/L?
No, and it is worth being clear about why. A result in ng/mL comes from an immunoassay that detects the protein, active or not. A result in U/L comes from an assay that measures catalysis. The ratio between them depends on how much of the circulating enzyme is functional, which is not a constant. There is no conversion factor.
What is the difference between total and prostatic acid phosphatase?
Total activity includes prostate, platelets, red cells, osteoclasts, liver, spleen and kidney. The prostatic fraction is the tartrate-inhibitable part of it. On one reagent insert at 37 °C the total interval is 0 to 5.1 U/L and the prostatic 0 to 1 U/L, so reading one against the other’s interval is a five-fold error.
Is prostatic acid phosphatase still used in prostate cancer?
Barely. Mayo Clinic Laboratories states it is no longer used to screen for or stage prostate cancer and that PSA gives more information. It survives in niche prognostic use and, more prominently, as the antigen in sipuleucel-T immunotherapy.
Is TRAP the same test as acid phosphatase?
No. Tartrate-resistant acid phosphatase isoform 5b is secreted only by osteoclasts and is measured on its own assay as a bone resorption marker; isoform 5a comes from macrophages and dendritic cells. The cytochemical TRAP stain in hairy cell leukaemia is a third thing again. None is obtained by subtracting the prostatic fraction from the total.
Why is my total acid phosphatase raised when nothing is wrong?
Haemolysis and delayed separation of the serum are the commonest causes: red cells and platelets are full of acid phosphatase and release it into the sample. Prostatic massage, catheterisation and recent instrumentation also raise it, as do bone disease and Gaucher disease.
Related calculators
References
- Mayo Clinic Laboratories. Prostatic Acid Phosphatase, Serum (PACP), test 8019. Reference value ≤2.1 ng/mL by chemiluminescent immunoassay.
- Labcorp. Prostatic Acid Phosphatase (PAP), test 004747. Reference interval 0–3.5 ng/mL by immunochemiluminometric assay.
- Cliniqa. Acid Phosphatase Reagent package insert PNR85128/PNR85303: total 0–4.5 U/L and prostatic 0–0.8 U/L at 30 °C; 0–5.1 and 0–1.0 U/L at 37 °C.
- Halleen JM, Alatalo SL, Suominen H, et al. Tartrate-resistant acid phosphatase 5b: a novel serum marker of bone resorption. J Bone Miner Res. 2000;15(7):1337–1345.
- Tefferi A, Falini B, Tiacci E. BRAF V600E mutation in hairy cell leukemia: from bench to bedside. Blood. 2016;128(15):1918–1927.
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.