Quantitative AP Threshold
Definition
A numerical activity level, expressed in units per litre of extract, above which a result is treated as semen-positive. Common thresholds range from 50 U/L (some US labs) to 500 U/L (historical UK FSS standard), reflecting different balances between sensitivity and specificity.
- Unit
- Units per litre of extract
- US example threshold
- 50 U/L, some laboratories
- Historical UK threshold
- 500 U/L, former FSS standard
- Purpose
- Separates semen-positive from negative acid phosphatase results
Common questions
Why do different forensic laboratories use different quantitative AP thresholds?+
Acid phosphatase is present at lower levels in other body fluids and vaginal secretions, so each laboratory sets its threshold based on its own validation data balancing the risk of a false negative against the risk of a false positive from non-seminal sources.
Does a result below the quantitative AP threshold rule out the presence of semen?+
Not definitively. A result below threshold is treated as AP-negative for screening purposes, but acid phosphatase activity can be reduced by degradation, dilution, or a low-volume deposit, so a negative screen is typically followed by confirmatory testing such as microscopy or a semen-specific marker before excluding semen entirely.
Related terms
- Acid Phosphatase (AP)
- An enzyme present at high concentrations in seminal plasma. The Brentamine Fast Violet B (or SERI AP) spray test detects acid phosphatase...
- Brentamine Fast Violet B Test
- A two-component spot test in which sodium alpha-naphthyl phosphate (substrate) is hydrolysed by AP to alpha-naphthol, which couples with fast violet B...
- False Positive
- A test result that indicates the presence of a target analyte when it is absent. In forensic serology this may mean incorrectly...
- Fluorescent AP Assay
- A more sensitive format using 4-methylumbelliferyl phosphate (4-MUP) as substrate. AP cleaves 4-MUP to 4-methylumbelliferone, which fluoresces under UV. Useful on aged...
- Prostatic Acid Phosphatase (PAP)
- The AP isoform secreted by acinar epithelial cells of the prostate gland, found in seminal plasma at concentrations of 25-1,000 U/mL, orders...