Prostate-Specific Antigen (PSA) is a protein produced primarily by both normal and malignant cells of the prostate gland. Following curative treatment for prostate cancer, such as a radical prostatectomy or radiation therapy, monitoring PSA levels becomes the primary method of surveillance. The goal of these treatments is to remove or destroy all cancer cells, which should cause the patient’s PSA level to drop significantly. A consistently low PSA level offers reassurance that the treatment has been effective. Understanding what these measurements mean is important for patients undergoing surveillance.
Defining Undetectable PSA
The term “undetectable PSA” is a clinical concept that does not mean the PSA level is mathematically zero. Instead, it signifies that the amount of prostate-specific antigen circulating in the blood is so minute that it falls below the lowest detection limit of the laboratory test being used. For many years, the standard lower limit of detection for PSA tests was around 0.1 nanograms per milliliter (ng/mL) or sometimes 0.2 ng/mL. Any reading below that threshold was simply reported as “undetectable.”
Achieving and maintaining an undetectable PSA level is the desired outcome following radical prostatectomy, where the entire prostate gland, the main source of PSA, is removed. For patients who receive radiation therapy, the PSA level usually drops much more slowly over a period of months or even years. In the case of radiation, the PSA level may never reach an undetectable level because some healthy, PSA-producing prostate tissue remains. In both scenarios, an undetectable status indicates a highly favorable response to the initial treatment.
The Role of Assay Sensitivity and the 0.02 Measurement
The interpretation of a 0.02 ng/mL reading hinges entirely on the sensitivity of the specific laboratory test performed. Conventional PSA assays have a lower limit of detection that is typically higher, meaning they cannot measure PSA levels below 0.1 ng/mL. Ultrasensitive PSA assays, often referred to as high-sensitivity tests, have been developed to measure these extremely low concentrations, sometimes down to 0.003 ng/mL. These modern ultrasensitive tests provide a specific number like 0.02 ng/mL, rather than simply reporting “<0.1" or "<0.05" ng/mL. Therefore, a PSA measurement of 0.02 ng/mL is technically a detectable level when measured by an ultrasensitive assay. In the clinical context of post-treatment monitoring, this reading is often still considered within the range of an excellent, near-undetectable result, particularly after a radical prostatectomy. Many clinicians define an undetectable or very low-detectable PSA as anything below 0.03 ng/mL or 0.05 ng/mL. A 0.02 ng/mL result is far below the threshold that triggers concern for recurrence. The significance of this low, measurable number lies in its trend over time. A stable 0.02 ng/mL measurement over multiple tests is generally viewed as equivalent to a clinically undetectable status, suggesting that the amount of remaining PSA-producing tissue, whether benign or malignant, is inert. If a patient's PSA consistently remains at 0.02 ng/mL, there is usually no immediate cause for alarm or change in the surveillance plan. The ultrasensitive test provides an early warning system, allowing physicians to track the exact number rather than waiting for it to cross an older, higher detection limit.
Understanding Biochemical Recurrence
A single, stable reading of 0.02 ng/mL is not considered a sign of treatment failure; failure is defined by a significantly higher threshold known as Biochemical Recurrence (BCR). BCR is the point at which the PSA level rises to a point that indicates the likely presence of cancer cells that survived the initial treatment. The definition of BCR depends on the type of treatment the patient received.
Following a radical prostatectomy, the most widely accepted clinical definition for BCR is a confirmed PSA level of 0.2 ng/mL or greater. This threshold is ten times higher than a 0.02 ng/mL reading.
For patients treated with radiation therapy, the definition for BCR is different because the prostate gland remains in place. Failure is defined by the Phoenix criterion, which is a rise of 2.0 ng/mL above the patient’s PSA nadir (the lowest point the PSA dropped to after radiation). A stable 0.02 ng/mL is lower than the levels that would initiate a discussion about follow-up treatment, such as salvage radiation or other therapies. The trend of the PSA level, specifically how quickly it is rising, is far more important than a single, ultra-low measurement.