Unit Converter
Acid phosphatase (ACP)
Synonyms
- ACP
- Total Acid Phosphatase
- Tartrate-Labile Acid Phosphatase
- Tartrate-Resistant Acid Phosphatase (TRAP)
- Prostatic Acid Phosphatase (PAP)
Units of Measurement
nkat/L, µkat/L, nmol/(s·L), µmol/(s·L), U/L, IU/L, µmol/(min·L), µmol/(h·L), µmol/(h·mL)
Description
Acid phosphatase (ACP) is a group of lysosomal enzymes that hydrolyze phosphate esters at an acidic pH (4.5–6.0).
It is found in:
- Prostate gland (highest concentration)
- Liver
- Spleen
- Red blood cells
- Platelets
- Bone (osteoclast-derived TRAP)
Historically, ACP was a major tumor marker for prostate cancer, but has now largely been replaced by PSA.
However, ACP still has clinical utility in certain conditions like metastatic prostate cancer, bone disorders, Gaucher disease, and rape/forensic evidence testing.
Physiological Role
ACP is involved in:
- Lysosomal degradation pathways
- Bone resorption (TRAP isoenzyme)
- Phosphate mobilization
- Cellular turnover processes
Osteoclasts release TRAP, making serum ACP useful in assessing certain bone diseases.
Clinical Significance
Elevated ACP
Occurs in:
1. Prostate Pathology
- Metastatic prostate carcinoma
- Locally advanced prostate cancer
- Rarely benign conditions (prostatitis, retention)
Prostatic ACP decreases rapidly after prostate surgery.
2. Bone Disorders
- Paget disease
- Hyperparathyroidism
- Bone metastases
- Osteolytic lesions
3. Hematologic & Storage Diseases
- Gaucher disease (markedly elevated)
- Niemann-Pick disease
4. Hepatic Conditions
Mild elevations in:
- Hepatic congestion
- Cirrhosis
5. Forensic/Medico-Legal Use
ACP is abundant in semen, historically used for detecting sexual assault evidence.
Low ACP
Not usually clinically significant.
May be seen in:
- Hypothyroidism
- Malnutrition
Reference Intervals
(Tietz 8E + IFCC + Mayo verified)
Adult Total ACP
- < 5 U/L (typical laboratory cutoff)
- SI equivalent: < 83 nkat/L
Prostatic ACP (PAP)
- < 2.0 U/L
Tartrate-Resistant Acid Phosphatase (TRAP)
- 1.5–5.0 U/L (laboratory dependent)
Note: Values vary significantly by method and substrate used.
Sample Handling – Critical
ACP is highly unstable in serum.
- Serum should be separated immediately.
- Samples must be kept refrigerated and analyzed quickly.
- Hemolysis falsely increases ACP (RBCs contain ACP).
- Avoid delay >1 hour for accurate prostatic ACP measurement.
Isoenzymes of ACP
ACP has five major isoenzymes:
| Isoenzyme | Source | Clinical Use |
| ACP1 | Prostate | Prostate cancer monitoring (historic) |
| ACP2 | Liver | Mild elevations in liver disease |
| ACP3 | Spleen | Storage disorders |
| ACP4 | Bone (TRAP) | Osteoclast activity, bone turnover |
| ACP5 | RBCs | Elevated in hemolysis |
ACP3 markedly elevated in Gaucher disease → useful diagnostic clue.
Unit Meanings
| Unit | Meaning |
| nkat/L | nanokatal per liter (SI) |
| µkat/L | microkatal per liter |
| nmol/(s·L) | nanomole per second per liter |
| µmol/(s·L) | micromole per second per liter |
| U/L or IU/L | enzyme units per liter |
| µmol/(min·L) | micromole per minute per liter |
| µmol/(h·L) | micromole per hour per liter |
| µmol/(h·mL) | micromole per hour per milliliter |
Clinical Pearls
- ACP rises early in metastatic prostate cancer but PSA is now the preferred marker.
- TRAP remains clinically relevant in Gaucher disease and bone turnover.
- Markedly elevated ACP with hepatosplenomegaly → think Gaucher disease.
- Hemolysis and delayed processing falsely elevate ACP.
- ACP is useful in evaluating rape cases (high levels in semen).
Interesting Fact
Before PSA was introduced in the 1980s, ACP was the primary laboratory marker for prostate cancer. It dramatically changed the landscape of urologic oncology before being replaced due to better sensitivity and stability.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Acid Phosphatase & Enzyme Chapter.
- IFCC Recommendation for Enzyme Measurement Standards.
- Mayo Clinic Laboratories - Acid Phosphatase (ACP) Test Catalog.
- ARUP Consult - ACP isoenzymes & TRAP testing.
- NCI (National Cancer Institute) - Prostatic ACP history and use.
- MedlinePlus / NIH - ACP overview.