Prostate-Specific Antigen (PSA) is a protein produced primarily by the prostate gland. Measuring PSA in the blood is a standard tool for diagnosing and monitoring prostate cancer. After definitive treatment, the goal of monitoring shifts from screening to surveillance. An “undetectable” PSA level becomes the benchmark for treatment success, indicating that the source of the protein—both cancerous and healthy prostate cells—has been successfully eliminated or suppressed. This article defines what “undetectable” means in the context of post-treatment monitoring following procedures like radical prostatectomy or radiation therapy.
Understanding PSA Measurement
The PSA test measures the protein concentration in the blood, reported in nanograms per milliliter (ng/mL). After the prostate gland is treated or removed, the interpretation of the PSA value changes entirely. Following curative therapy, the primary objective is to eliminate the tissue that produces PSA.
Post-treatment PSA measurement is no longer a screening tool but a highly sensitive indicator of residual or recurrent disease. The target is an extremely low value, reflecting the absence of prostate tissue. The definition of what constitutes an “undetectable” reading depends heavily on the specific treatment received.
Defining the “Undetectable” Threshold
The definition of “undetectable” is most stringent following a radical prostatectomy (surgical removal of the entire prostate gland). Since the major source of PSA is removed, the protein level is expected to fall rapidly within six to eight weeks. For these patients, “undetectable” is generally defined as a PSA level below the minimum measurable limit of the assay, often less than 0.1 ng/mL or 0.05 ng/mL. This value signifies that the amount of PSA present is too minute for the testing technology to reliably quantify, not a true zero.
The threshold differs for patients who undergo radiation therapy, such as external beam radiation or brachytherapy, because the prostate gland remains in the body. Healthy prostate cells continue to produce small amounts of PSA, meaning the level rarely becomes truly undetectable. The goal is for the PSA to drop to its lowest point, known as the nadir, often below 1.0 ng/mL. An undetectable result after radiation is uncommon and typically defined as a PSA nadir below 0.5 ng/mL. Surveillance for radiation patients focuses on stability and any significant rise above this lowest point.
Post-Treatment Monitoring Schedule
Regular PSA testing is the principal method of surveillance following definitive treatment. The frequency is determined by the patient’s risk profile and time since therapy. Testing typically begins a few months after treatment to allow the PSA level to stabilize to its new baseline.
For both surgery and radiation patients, the standard schedule involves checks every three to six months for the first few years. If the PSA remains stable and low after two to five years, the frequency often decreases to an annual test. Surveillance often uses an ultra-sensitive PSA test, which can measure levels down to 0.01 ng/mL or lower. This high sensitivity is necessary to detect the earliest possible sign of recurrence while minimizing patient anxiety from frequent testing.
When the PSA Rises: Biochemical Recurrence
When the PSA level moves above the established undetectable threshold, it is termed “Biochemical Recurrence” (BCR). BCR indicates that prostate cancer cells have survived the initial treatment or have returned, producing measurable amounts of the protein. The specific numerical criteria for defining BCR vary based on the initial treatment.
For men who underwent radical prostatectomy, the consensus definition for BCR is a PSA level of 0.2 ng/mL or higher, confirmed by a second measurement. This low threshold is used because any measurable PSA after the prostate is removed suggests residual tissue.
For patients treated with radiation therapy, BCR is defined differently, usually as a rise of 2.0 ng/mL above the patient’s nadir (lowest post-treatment PSA level). BCR is a serious finding that necessitates further investigation and treatment planning, but it is distinct from clinical recurrence, which involves symptoms or visible spread of the cancer on imaging.