Introduction
- Prostate Specific Antigen (PSA) is an important tumor marker used to evaluate prostate diseases.
- It is a glycoprotein enzyme produced mainly by the epithelial cells of the prostate gland.
- PSA is normally secreted into seminal fluid, where it helps liquefy semen.
- Only a small amount of PSA is present in the blood of healthy men.
- Elevated PSA levels may indicate prostate cancer, benign prostatic hyperplasia (BPH), or prostatitis.
- PSA testing is widely used for the screening, diagnosis, monitoring, and follow-up of prostate diseases.
- PSA results should always be interpreted along with clinical examination and other diagnostic tests.
Definition of Prostate-Specific Antigen (PSA)
- Prostate-Specific Antigen (PSA) is a serine protease glycoprotein enzyme produced mainly by the prostate gland.
- It belongs to the kallikrein family (KLK3).
- Its primary function is to liquefy semen after ejaculation.
- PSA is present in high concentration in seminal fluid and low concentration in blood.
- It is an important biomarker for prostate disorders, especially prostate cancer.
Structure and Characteristics
- PSA is a single-chain glycoprotein.
- It contains 237 amino acids.
- Molecular weight is approximately 33–34 kDa.
- It belongs to the kallikrein (KLK3) family.
- Functions as a serine protease enzyme.
- Produced mainly by the epithelial cells of the prostate gland.
- Found mainly in seminal fluid with a small amount in blood.
- Circulates in serum as free PSA and complexed PSA.
Synthesis and Regulation
- PSA is synthesized by the secretory epithelial cells of the prostate gland.
- It is encoded by the KLK3 gene on chromosome 19.
- PSA production is stimulated by testosterone and dihydrotestosterone (DHT).
- After synthesis, PSA is secreted into the prostatic ducts and seminal fluid.
- Normally, only a small amount enters the bloodstream.
- Prostate diseases increase the leakage of PSA into the blood, resulting in elevated serum PSA levels.
Biological Functions
- Liquefies semen after ejaculation.
- Reduces the viscosity of seminal fluid.
- Improves sperm motility.
- Facilitates sperm transport.
- Supports male fertility.
- Maintains normal prostate gland function.
- Acts as an important tumor marker for prostate diseases.
- Helps in the diagnosis and monitoring of prostate cancer.
Types of PSA
PSA circulates in the blood in different forms. The main types of PSA used in clinical practice are Total PSA (tPSA), Free PSA (fPSA), and Complexed PSA (cPSA).
1. Total PSA (tPSA)
- Total PSA is the sum of free PSA and complexed PSA present in the blood.
- It is the most commonly measured PSA test.
- Used for the screening, diagnosis, and monitoring of prostate diseases.
- Elevated levels may be seen in:
- Prostate cancer
- Benign prostatic hyperplasia (BPH)
- Prostatitis
2. Free PSA (fPSA)
- Free PSA circulates in the blood without binding to plasma proteins.
- It usually represents 5–35% of the total PSA.
- The free PSA/total PSA ratio helps differentiate prostate cancer from benign conditions.
- A lower free PSA percentage is associated with a higher risk of prostate cancer.
3. Complexed PSA (cPSA)
- Complexed PSA is bound to plasma proteins, mainly α1-antichymotrypsin (ACT).
- It constitutes the major portion of total PSA in the blood.
- Complexed PSA levels are generally higher in patients with prostate cancer.
- It can improve the specificity of prostate cancer detection in some patients.
Normal Reference Range
The normal PSA level varies with age, but the commonly accepted reference range is:
| PSA Level | Interpretation |
|---|---|
| < 4.0 ng/mL | Normal |
| 4–10 ng/mL | Borderline (Gray Zone); further evaluation may be required |
| > 10 ng/mL | High risk of prostate cancer |
Age-Specific Reference Range
| Age (Years) | Normal PSA (ng/mL) |
|---|---|
| 40–49 | 0–2.5 |
| 50–59 | 0–3.5 |
| 60–69 | 0–4.5 |
| 70–79 | 0–6.5 |
Note: Reference ranges may vary slightly depending on the laboratory and analytical method used.
Indications for PSA Testing
PSA testing is indicated for:
- Screening for prostate cancer in high-risk men.
- Diagnosis of prostate disorders.
- Evaluation of patients with urinary symptoms.
- Monitoring response to prostate cancer treatment.
- Detection of recurrence after prostate surgery or radiotherapy.
- Follow-up of patients with benign prostatic hyperplasia (BPH).
- Assessment of prostatitis.
Specimen Requirements
- Specimen: Serum
- Sample Collection: Venous blood (3–5 mL)
- Container: Plain (red-top) tube or serum separator tube (SST)
- Sample Condition: Clear, non-hemolyzed serum
- Storage: Store at 2–8°C if analyzed within 48 hours; freeze at −20°C for longer storage.
- Avoid sample collection immediately after digital rectal examination (DRE), prostate biopsy, catheterization, or ejaculation, as these procedures may temporarily increase PSA levels.
Methods for PSA Estimation
The following laboratory methods are commonly used for PSA estimation:
- Enzyme-Linked Immunosorbent Assay (ELISA)
- Chemiluminescent Immunoassay (CLIA) (most commonly used)
- Electrochemiluminescence Immunoassay (ECLIA)
- Fluorescence Immunoassay (FIA)
- Radioimmunoassay (RIA) (rarely used today)
Most Preferred Method
Chemiluminescent Immunoassay (CLIA) is the preferred method because it offers:
- High sensitivity
- High specificity
- Rapid analysis
- Excellent accuracy
- Suitable for routine clinical laboratories
Principle of PSA Test
Most PSA tests are based on the immunoassay principle, particularly the Chemiluminescent Immunoassay (CLIA).
- PSA present in the patient’s serum acts as an antigen.
- It binds specifically to anti-PSA antibodies.
- An antigen-antibody complex is formed.
- A labeled enzyme or chemiluminescent marker produces a measurable light signal.
- The intensity of the emitted light is directly proportional to the PSA concentration in the sample.
- The analyzer calculates the PSA level and reports the result in ng/mL.
Procedure for PSA Testing
- Collect 3–5 mL of venous blood in a plain or serum separator tube.
- Allow the blood to clot and separate the serum by centrifugation.
- Load the serum sample into the automated immunoassay analyzer.
- The analyzer mixes the sample with anti-PSA antibodies and detection reagents.
- An antigen-antibody reaction occurs.
- The emitted signal is measured by the analyzer.
- The PSA concentration is calculated automatically.
- Report the result in ng/mL.
Interpretation of PSA Results
| PSA Level (ng/mL) | Interpretation |
|---|---|
| < 4.0 | Normal |
| 4–10 | Borderline (Gray Zone); further evaluation recommended |
| > 10 | High suspicion of prostate cancer |
| > 20 | Suggests advanced prostate disease or metastatic cancer |
Causes of Increased PSA Levels
PSA levels may increase in both malignant and benign conditions.
Malignant Causes
- Prostate cancer
- Metastatic prostate cancer
- Recurrent prostate cancer after treatment
Benign Causes
- Benign prostatic hyperplasia (BPH)
- Prostatitis (inflammation of the prostate)
- Urinary tract infection (UTI)
- Acute urinary retention
- Recent ejaculation (within 24–48 hours)
- Digital rectal examination (DRE)
- Prostate biopsy or catheterization
- Aging (PSA increases with age)
Causes of Decreased PSA Levels
PSA levels may decrease due to:
- Successful treatment of prostate cancer
- Radical prostatectomy (removal of the prostate gland)
- Radiation therapy
- Hormonal (androgen deprivation) therapy
- Use of 5-alpha reductase inhibitors (e.g., Finasteride, Dutasteride)
- Small prostate gland
Clinical Applications of PSA
PSA testing is widely used in the diagnosis and management of prostate diseases.
- Screening for prostate cancer in high-risk men.
- Early detection of prostate cancer.
- Differentiation of prostate cancer from benign prostatic hyperplasia (BPH).
- Monitoring response to prostate cancer treatment.
- Detection of recurrence after surgery or radiotherapy.
- Follow-up of patients with prostate cancer.
- Assessment of patients with urinary symptoms.
- Evaluation of prostatitis and other prostate disorders.