Prostate-Specific Antigen (PSA) is a protein produced primarily by cells in the prostate gland. Measuring this protein via a blood test is a common tool in men’s health. Physicians order a PSA test for screening for prostate cancer, monitoring the effectiveness of cancer treatment, or checking for disease recurrence. While high PSA levels may indicate cancer, they can also signal non-cancerous conditions like an enlarged prostate or inflammation.
Why Serum is Required for PSA Testing
The laboratory test for PSA measures the amount of the protein circulating in the bloodstream. For most diagnostic assays, laboratories prefer to test serum rather than plasma. Serum is the liquid portion of the blood that remains after the blood has been allowed to clot, with the clotting factors and cells separated out.
Plasma is the liquid portion of blood obtained when an anticoagulant is added to prevent clotting. Anticoagulants, such as EDTA or citrate, can often interfere with the sensitive chemical reactions of the PSA assay, leading to inaccurate or unreliable results. Because PSA is a protein, it is measured most accurately when the blood sample is processed naturally into serum.
Identifying the Correct Blood Collection Tube
The most common blood collection tube used to obtain a serum sample for PSA testing has a Gold top. This tube is known as a Serum Separator Tube (SST) and is the preferred choice in many clinical settings. The Gold top tube contains two key components: a clot activator and an inert separation gel.
The clot activator speeds up the natural clotting process of the blood. Once the blood has clotted, the tube is spun in a centrifuge, and the separation gel forms a physical barrier between the heavy blood cells and the light, clear serum. This barrier prevents the serum from mixing with the cells, which is crucial because blood cells can release substances that interfere with the PSA measurement over time.
A plain Red top tube, which contains no additives, is also acceptable for PSA testing, but it requires the laboratory staff to manually separate the serum after clotting. Variations like a “Tiger Top” or a speckled red/gray top are often functionally identical to the Gold SST, as they are all designed to yield a clean serum sample.
Patient Preparation for the PSA Test
Accurate PSA results depend heavily on proper patient preparation before the blood draw. Certain activities that involve manipulation or irritation of the prostate gland can cause the temporary release of PSA into the bloodstream, leading to falsely elevated results.
Preparation Requirements
Patients must avoid ejaculation for a specified period, typically 24 to 48 hours, before the test. Vigorous physical activities and high-impact exercise, such as cycling, must also be avoided for at least 48 hours. Furthermore, any recent medical procedure involving the prostate, such as a Digital Rectal Exam (DRE) or a prostate biopsy, can significantly raise the PSA reading.
A blood draw should ideally be scheduled before a DRE. If a prostate biopsy has been performed, a period of up to three weeks or more may be necessary to allow the PSA level to stabilize.