Understanding PSA Terminology: Serum PSA vs. Free PSA vs. PSA "Titre"
The terms "serum PSA" and "total PSA" (tPSA) are synonymous and refer to the total amount of prostate-specific antigen measured in blood, while "free PSA" (fPSA) refers specifically to the unbound fraction of PSA not complexed to proteins—the term "titre" is not standard medical terminology in PSA testing and likely represents a misunderstanding or mistranslation of these measurements.
Core Definitions
Total/Serum PSA (tPSA)
- What it measures: The sum of all PSA forms circulating in blood 1
- Composition: Consists of free PSA (10-40%) plus PSA bound to protease inhibitors, primarily alpha-1-antichymotrypsin (60-90%) 2, 1
- Clinical use: Primary screening tool for prostate cancer detection, with traditional threshold of 4.0 ng/mL 3
Free PSA (fPSA)
- What it measures: Only the unbound, uncomplexed PSA molecules 1
- Key characteristic: Cleared by kidneys (unlike complexed PSA which is cleared by liver) 2
- Clinical significance: Men with prostate cancer have proportionally less free PSA than men with benign conditions 1
The Free-to-Total PSA Ratio (%fPSA)
This ratio is the clinically meaningful measurement, not the absolute free PSA value alone:
- FDA-approved use: For men with total PSA between 4-10 ng/mL (the "diagnostic gray zone") 1, 4
- Cancer discrimination: Lower ratios suggest higher cancer probability—men with prostate cancer typically have %fPSA <15-25%, while those with BPH have higher ratios 1
- Clinical threshold: A 25% fPSA cutoff detects 95% of prostate cancers while avoiding 20% of unnecessary biopsies 1, 4
- Age-independent: Unlike total PSA, the %fPSA ratio does not correlate with age 5
About "PSA Titre"
This term does not exist in standard medical literature or clinical practice. The evidence provided contains no references to "PSA titre" or "serum PSA titre." This likely represents:
- Confusion with "titer": A term used in immunology for antibody concentrations, not applicable to PSA
- Mistranslation: From another language where PSA measurements might be described differently
- Misunderstanding: Of the terms "total PSA" or "PSA level"
Clinical Application Algorithm
When interpreting PSA results:
- Start with total PSA measurement in ng/mL (not "titre")
- If total PSA is 4-10 ng/mL with normal DRE: Order free PSA to calculate %fPSA ratio 1
- Calculate %fPSA: (free PSA ÷ total PSA) × 100
- Interpret the ratio:
- %fPSA <15%: High cancer probability, strongly consider biopsy
- %fPSA 15-25%: Intermediate risk, individualize decision
- %fPSA >25%: Lower cancer probability, may avoid immediate biopsy 1
Important Caveats
Laboratory variability matters: Different PSA assays are not interchangeable and can vary by 20-25% 6. Always use the same laboratory and assay for serial monitoring.
Factors affecting measurements:
- Free PSA specifically: Altered by dialysis (increased %fPSA in dialysis patients due to impaired renal clearance) 2
- Total PSA: Affected by 5α-reductase inhibitors (finasteride/dutasteride reduce PSA by ~50%), prostatitis, recent ejaculation, prostate biopsy 2, 3, 6
Complexed PSA (cPSA): A third form measuring PSA bound to alpha-1-antichymotrypsin directly, with equivalent thresholds of 2.2 ng/mL (cPSA) = 2.5 ng/mL (tPSA) and 3.4 ng/mL (cPSA) = 4.0 ng/mL (tPSA), though not widely adopted in clinical practice 1, 4.