Acid Phosphatase (ACP) Test: Purpose, Colorimetric Method, Clinical Significance

The Acid Phosphatase (ACP) test is an enzyme assay used in the evaluation of prostate cancer (especially bone metastasis), bone disease, and hemolytic disorders. Although its use has declined with the adoption of PSA, ACP still holds value in specific clinical contexts.

1. What Is Acid Phosphatase (ACP)?

Acid Phosphatase is an enzyme that hydrolyzes phosphate esters in an acidic environment (pH 4–6). It is present in multiple tissues, including:

• Prostate (Prostatic ACP)
• Liver
• Spleen
• Red blood cells (RBCs)
• Bone (osteoclast-related)

Historically, Prostatic ACP was used as a marker for prostate cancer, but PSA has largely replaced it due to higher sensitivity and specificity. Still, ACP remains useful in certain diagnostic situations.

2. Purpose of the ACP Test

• Evaluation of prostate cancer — primarily to assess bone metastasis; also used as a postoperative monitoring tool.
• Assessment of hemolytic disorders — RBCs contain ACP; levels may rise with hemolysis.
• Bone disease evaluation — Paget disease, bone metastasis, and conditions with increased bone resorption.
• Liver and spleen disorders — mild elevation may occur but lacks specificity.

3. Test Method: Colorimetry

Most clinical laboratories use the colorimetric method for ACP measurement.

Principle of Colorimetry

ACP hydrolyzes a substrate under acidic conditions to produce a colored compound. The intensity of this color is measured using a spectrophotometer, typically at 405 nm, and is proportional to ACP activity.

Specimen

• Serum
• Hemolysis must be strictly avoided to prevent falsely elevated results.

Pre-analytical Considerations

• ACP is heat-labile → process or cool the sample immediately after collection.

4. Reference Range

Total ACP: < 6.6 U/L (ranges vary by laboratory and method)

5. Causes of Increased ACP

• Prostate cancer — markedly elevated in cases with bone metastasis.
• Bone diseases — Paget disease, giant cell tumors, bone metastasis.
• Hemolysis — due to high ACP content in RBCs.
• Liver and spleen disorders — mild, non-specific increase.
• Thrombocytosis — platelets contain ACP.

Note: Total ACP elevation may require differentiation using Prostatic ACP (PAcP) fraction.

6. Causes of Decreased ACP

• Malnutrition
• Hypothyroidism
• Rare hereditary enzyme deficiencies

7. Interpretation Tips

• PSA is superior for prostate cancer diagnosis; ACP is now mainly supportive.
• Highly sensitive to hemolysis → check hemolysis index.
• Not specific — must be interpreted with ALP, PSA, CBC, and clinical findings.
• Prostatic ACP fraction testing may be helpful in selected cases.

8. Summary Table

Item	Details
Test Name	Acid Phosphatase (ACP)
Method	Colorimetry
Specimen	Serum
Main Uses	Prostate cancer (metastasis), bone disease, hemolysis
Increase	Prostate cancer, Paget disease, hemolysis, bone metastasis
Cautions	Hemolysis-sensitive; PSA preferred

9. References

• Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 6th ed.
• Henry’s Clinical Diagnosis and Management by Laboratory Methods, 24th ed.
• Clinical Chemistry. Burtis CA, Ashwood ER.
• Campbell’s Urology, 9th ed.
• IFCC Guidelines for Enzyme Testing.