Prostate-specific antigen (PSA) is a protein made mainly in the prostate. A bit of PSA is released into the bloodstream during each ejaculation, and PSA in ejaculate makes it easier for the semen to carry sperm to the fallopian tubes during conception.
PSA can have two basic states. It may be bound to another protein or float freely.
PSA, a protein produced by prostate gland cells, circulates through the body in two ways: either bound to other proteins or on its own. PSA traveling alone is called free PSA. The free-PSA test measures the percentage of unbound PSA; the PSA test measures the total of both free and bound PSA.
Prostate cancer can raise PSA levels, but so can other conditions. These include an enlarged prostate, prostatitis, and advancing age. In fact, studies have shown that about 75% of men with an elevated PSA do not have prostate cancer. To determine which men actually have cancer and which don’t, physicians traditionally perform a biopsy. Undergoing a biopsy isn’t as traumatic as surgery, but it does cause discomfort and can provoke anxiety.
Rather than subject everyone with an elevated PSA to a biopsy, some urologists measure free PSA in patients with a total PSA level between 4 ng/ml and 10 ng/ml. Studies have shown that men with a total PSA in this “gray area” and a free PSA greater than 25% are more likely to have a benign condition (=not harmful in effect) than to have cancer, making a biopsy unnecessary. Men with a total PSA in the same range (between 4 ng/ml and 10 ng/ml) and a free PSA below 10% need to have a biopsy. More likely than not, they have prostate cancer.
Doctors interpret PSA results based on several factors, including:
Total PSA ranges may vary slightly. According to a review of studies, the normal ranges for total PSA in nanograms per milliliter (ng/mL) are:
age 49 or younger: 0.0–2.5 ng/mL
age 50 to 59: 0.0–3.5 ng/mL
age 60 to 69: 0.0–4.5 ng/mL
age 70 or older: 0.0–6.5 ng/mL
When total PSA levels are higher than the appropriate range, a person has an increased risk for prostate cancer. It does not show conclusively that a person has cancer. Doctors will then compare the results of this test with results of a free PSA test.
Free PSA ratio
A high total PSA level and a low free PSA level generally indicate a risk for prostate cancer.
Free PSA ranges can vary, but in general, a higher ratio of free PSA to total PSA is healthier.
When the ratio of free to total PSA is between 0 and 10 percent, the risk of having cancer may be as high as 56 percent. When this ratio is greater than 25 percent, this risk reduces to 8 percent.
However, these results are not conclusive evidence for or against a cancer diagnosis, and a doctor may still want to do a tissue biopsy.
Doctors will consider additional factors, such as a person’s age, ethnicity, and prostate volume. For instance, PSA levels normally rise with age, and this can affect the results of the tests.
Other factors, called PSA kinetics, may also influence how a doctor views the results in people who are currently undergoing treatment for prostate cancer.
PSA kinetics reflect how levels change over time. It can be difficult to obtain a clear picture, but kinetics can play an important role in the outlook and treatment of prostate cancer. The factors involved are PSA velocity and doubling time.
PSA velocity measures how rapidly levels are rising, and it can indicate that prostate cancer is recurring.
Doubling time is a calculation of how long it takes for PSA levels to double. A faster doubling time can suggest a more aggressive cancer, which can be a sign that the cancer is spreading.
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