Unit Converter
Prostate – specific antigen (PSA)
(Seminal Enzyme Released by Prostate - Key Marker for Prostate Cancer Screening, Diagnosis, Monitoring & Recurrence)
Synonyms
- PSA
- Prostate-specific antigen
- γ-seminoprotein
- Kallikrein-related peptidase 3 (KLK3)
- Total PSA (tPSA)
- Free PSA (fPSA)
Units of Measurement
- ng/mL
- ng/dL
- ng/100 mL
- ng%
- ng/L
- µg/L
Unit Relationships
1 ng/mL=1000 ng/L1\ \text{ng/mL} = 1000\ \text{ng/L}1 ng/mL=1000 ng/L 1 ng/mL=1 µg/L1\ \text{ng/mL} = 1\ \text{µg/L}1 ng/mL=1 µg/L 1 ng/mL=100 ng/dL1\ \text{ng/mL} = 100\ \text{ng/dL}1 ng/mL=100 ng/dL \text{ng%} = \text{ng/dL}
All units interconvert directly.
Description
PSA is a serine protease enzyme produced almost exclusively by prostate epithelial cells.
Normally, PSA is secreted into seminal fluid; only small amounts enter the bloodstream.
PSA is organ-specific but not cancer-specific, meaning:
- Elevated PSA → prostate abnormality
- Cause may be benign or malignant
PSA exists in blood as:
- Free PSA (fPSA)
- Complexed PSA (cPSA)
- Total PSA (tPSA = fPSA + cPSA)
Clinically most testing uses total PSA, with %fPSA used for cancer risk stratification.
Physiological Role
- Liquefies semen
- Enhances sperm motility
- Member of kallikrein protease family
Clinical Significance
HIGH PSA (Most Important Clinical Finding)
Elevated PSA can occur in:
1. Prostate Cancer
PSA >4 ng/mL increases suspicion.
Levels correlate with:
- Tumor volume
- Metastasis risk
- Recurrence risk
2. Benign Prostatic Hyperplasia (BPH)
Most common non-cancer cause.
3. Prostatitis / Infection
Can significantly elevate PSA (10–20+ ng/mL).
4. Prostatic Manipulation
- DRE (mild)
- Catheterization
- Cystoscopy
- TRUS biopsy (very high)
5. Ejaculation
Raises PSA for 24–48 hours.
6. Aging
PSA gradually increases.
7. UTI / Urinary retention
LOW PSA
- Normal finding
- After radical prostatectomy (should be undetectable)
- On 5-α reductase inhibitors (finasteride/dutasteride) → PSA falls ~50%
Reference Intervals
(Tietz 8E + AUA + NCCN + Mayo + ARUP)
Age-Adjusted Total PSA Ranges
| Age | Normal PSA (ng/mL) |
| 40–49 years | 0 – 2.5 |
| 50–59 years | 0 – 3.5 |
| 60–69 years | 0 – 4.5 |
| 70–79 years | 0 – 6.5 |
General Screening Cutoff
- >4.0 ng/mL → abnormal
- 2.5–4.0 ng/mL → borderline, risk increases with age
Cancer Risk by PSA Level
| PSA (ng/mL) | Approximate Cancer Risk |
| <1 | Very low |
| 1–3 | Low–moderate |
| 4–10 | ~25% risk |
| >10 | >50% risk |
| >20 | High risk / metastasis possible |
After Prostatectomy
- PSA should fall to <0.1 ng/mL
- Any rise → biochemical recurrence
Free PSA (%fPSA) Interpretation
%fPSA=Free PSATotal PSA×100\%\text{fPSA} = \frac{\text{Free PSA}}{\text{Total PSA}} \times 100%fPSA=Total PSAFree PSA×100
Used when total PSA is 4–10 ng/mL.
| %fPSA | Cancer Probability |
| <10% | High risk |
| 10–25% | Intermediate |
| >25% | Low risk |
Low %fPSA = higher likelihood of prostate cancer.
Diagnostic Uses
1. Prostate Cancer Screening
Men aged 45–75 depending on risk:
- African ancestry
- Family history
- BRCA2 mutation
- High-risk groups
2. Diagnosis & Risk Stratification
PSA used with:
- DRE
- MRI prostate
- Biopsy
- Free/total ratio
- PSA density (PSAD)
- PSA velocity
3. Monitoring After Treatment
Radical prostatectomy:
- PSA should become undetectable (<0.1 ng/mL)
Radiation therapy:
- Nadir PSA +2 ng/mL = biochemical recurrence
4. Monitoring Active Surveillance
Trend over time more important than a single value.
5. Evaluation of BPH & Prostatitis
Analytical Notes
- Avoid ejaculation for 48 hours before testing
- Delay PSA measurement for:
- 6 weeks after prostatitis
- 1–2 weeks after DRE
- 4–6 weeks after UTI
- 6 weeks after biopsy
- 6 weeks after prostatitis
- Hemolysis minimal effect
- 5-α reductase inhibitors cut PSA in half—always double the result
Clinical Pearls
- PSA is organ-specific, not cancer-specific.
- Rapid PSA doubling time suggests aggressive cancer.
- MRI prior to biopsy improves cancer detection.
- High PSA with normal imaging may still indicate clinically significant cancer.
- Low %fPSA increases cancer probability even when PSA is borderline.
- PSA >100 ng/mL almost always indicates metastatic prostate cancer.
Interesting Fact
PSA was originally discovered as a seminal biomarker for forensic identification before becoming one of the most important cancer markers in medicine.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Tumor Markers & PSA
- AUA Guidelines - Early Detection of Prostate Cancer
- NCCN Guidelines - Prostate Cancer Detection & Management
- EAU Prostate Cancer Guidelines
- Mayo Clinic Laboratories - PSA
- ARUP Consult - Prostate Cancer Testing
- NIH / MedlinePlus - PSA Test