The Prostate-Specific Antigen (PSA) test is a standard blood analysis that measures a protein produced primarily by the prostate gland. PSA circulates in the bloodstream and serves as a biochemical marker for prostate health. The test is used for screening potential issues and monitoring patients with conditions like prostate cancer or benign prostatic hyperplasia. A result outside the expected range signals the need for further investigation, helping physicians evaluate the prostate’s state.
The Required Collection Tube for PSA Testing
The PSA test requires serum, the liquid component of blood remaining after clotting. Serum is preferred because the assay is designed to measure PSA in this matrix, and additives in other tubes can interfere with the chemical reaction. The most commonly used collection tube is the Serum Separator Tube (SST), typically identified by a Gold or Tiger Top stopper. This tube contains a clot activator and a gel barrier to ensure proper clotting and separation of the serum.
The gel forms a physical barrier between the heavier blood cells and the lighter serum following centrifugation. Some laboratories may accept a plain Red Top tube, which contains no additives, allowing the blood to clot naturally. Both tubes facilitate serum production without introducing anticoagulants, which are the main concern for assay interference. Tubes containing anticoagulants, such as the Lavender Top (EDTA) or Light Blue Top (Sodium Citrate), are generally unacceptable because these chemicals compromise the accuracy of the PSA measurement.
Critical Patient Preparation Factors
Obtaining a reliable PSA result depends heavily on the patient’s activities leading up to the blood draw. Physical manipulation or stimulation of the prostate can cause a temporary, significant increase in PSA levels, leading to a falsely elevated reading.
Activities to Avoid
Patients must abstain from sexual activity and ejaculation for at least 48 hours before the test, as this can temporarily raise the PSA concentration. Strenuous physical activities, especially prolonged bicycling that puts pressure on the perineum, should also be avoided for at least 48 hours before collection.
Medical Procedures and Medications
Recent medical procedures involving the prostate must also be considered. A Digital Rectal Exam (DRE) should ideally be performed after the blood is drawn, as the physical examination can cause a transient spike in PSA. More invasive procedures require a longer waiting period; for example, testing must be delayed for approximately six weeks following a prostate biopsy to ensure accuracy. Patients must also inform their healthcare provider about medications, such as 5-alpha reductase inhibitors (like finasteride or dutasteride), as these drugs are known to lower the measured PSA value.
Specimen Handling and Processing Steps
The integrity of the PSA specimen relies on precise handling immediately following collection. Once blood is drawn into the serum tube, it must be allowed to clot completely at room temperature, typically taking 30 minutes to one hour. This clotting separates the fibrin and cellular components from the liquid serum. Failure to allow adequate clotting time can result in fibrin strands forming later, which interferes with the testing equipment.
After clotting, the tube is placed into a centrifuge and spun at high speed to separate the serum from the blood cells and the clot. The serum should be separated promptly, ideally within 60 minutes to two hours of collection. The separated serum is then transferred into a clean, labeled plastic transport tube, a process known as aliquoting.
This prompt separation is important because prolonged contact between the serum and blood cells can alter analyte concentration. Improper handling, such as vigorous shaking, can cause hemolysis (the rupture of red blood cells). Hemolysis releases cellular components into the serum, interfering with the PSA assay and potentially leading to sample rejection. For short delays, the serum can be refrigerated; for long-term storage, the specimen must be frozen to maintain PSA protein stability.