Prostate-Specific Antigen (PSA) is a protein produced primarily by the cells of the prostate gland. Monitoring this protein is the main method of surveillance after definitive treatment for prostate cancer, such as radical prostatectomy or radiation therapy. The goal after treatment is for the PSA level in the blood to drop to the lowest possible level, reflecting the successful removal or destruction of the cancer cells. Following treatment, the focus shifts from screening for new cancer to monitoring for the persistence or return of the disease.
Defining “Undetectable” PSA
The term “undetectable” is a technical one, referring to the sensitivity of the laboratory equipment, not a clinical judgment. It means the amount of PSA present is below the lowest level the specific testing assay can reliably measure, known as the lower limit of quantification (LoQ) or limit of detection (LoD). Different laboratories use various assays, especially high-sensitivity tests (ultrasensitive PSA assays), meaning the technical definition of “undetectable” can vary. For instance, a standard assay might report anything below 0.1 nanograms per milliliter (ng/mL) as undetectable. Ultrasensitive tests, commonly used for post-treatment monitoring, can reliably measure levels much lower, sometimes down to 0.01 ng/mL.
Clinical Interpretation of 0.02 ng/mL
A PSA reading of 0.02 ng/mL is a very low number that indicates a highly successful initial response to treatment. While technically a “detectable” result because it is above zero, it is far below the level considered worrisome by clinicians. Following a radical prostatectomy, 0.02 ng/mL is often classified clinically as “effectively undetectable.” This small, measurable amount may be produced by tiny amounts of non-cancerous prostate tissue remaining after surgery or from benign laboratory variability. Some studies even use a cutoff of 0.02 ng/mL or 0.05 ng/mL to define an undetectable status, demonstrating its clinical acceptability.
Thresholds for Biochemical Recurrence
A single, low-level PSA reading like 0.02 ng/mL must be contrasted with the established thresholds that define a treatment failure. The return of measurable PSA following definitive local therapy is known as Biochemical Recurrence (BCR). For patients who have undergone a radical prostatectomy, the American Urological Association (AUA) defines BCR as a confirmed PSA level of 0.2 ng/mL or greater. This requires two consecutive tests showing a PSA value at or above this level to confirm recurrence. For patients treated with radiation therapy, BCR is defined as a rise in PSA of 2 ng/mL above the lowest point achieved after treatment (the nadir).
Monitoring Trends and Lab Variability
For patients managing prostate cancer surveillance, the stability and trend of the PSA level over time are far more significant than any single measurement. A stable PSA of 0.02 ng/mL over multiple years is fundamentally different from a rising pattern, such as 0.02 ng/mL, 0.04 ng/mL, and then 0.06 ng/mL in subsequent tests. This rate of change, known as PSA velocity, guides clinical decisions. Minor fluctuations in these very low numbers are common and can be attributed to benign biological variability or differences in laboratory testing. Therefore, it is often recommended that patients use the same laboratory for consistent monitoring to minimize inter-laboratory variability.