What is a PSA Test?
PSA stands for “prostate-specific antigen”
This is a glycoprotein that acts like an enzyme, a protease, produced almost exclusively by prostate epithelial cells whose function consists of liquefying the sperm, i.e., making it liquid after the ejaculated sperm has “clotted”.
It therefore exists in the sperm and in the prostate cells (both malignant and benign) and in the peripheral blood.
It is thus possible to gauge the PSA levels in the blood via a simple blood test. The results obtained help to establish whether the case involves a benign prostate enlargement or prostate cancer. If cancer is suspected, a prostate biopsy must be taken.
Importance of the PSA Test Results
A high PSA score implies screening for prostate cancer, since the patient may have cancer.
However, besides cancer, other situations can be responsible for a high score. For example:
- Prostatitis (acute or chronic);
- Urinary or genital infection;
- Occurrence of an episode of urinary retention (“trapped urine”);
- Benign prostate hyperplasia (BPH);
- Realisation of an endoscopy, biopsy or prostate surgery.
Classically, a PSA score of 4 (the dosage unit is ng/ml, i.e., nanograms per millimetre) was considered suspicious.
A score lower than 4 did not need a biopsy, a score higher than 10 was suspicious and anything between 4 and 10 was in the so-called “grey zone”.
Today, things have changed, and any score must be framed within a series of other parameters which the urologist assesses for each patient.
The same PSA score in different people can mean something different depending on other variables – e.g., a palpable nodule during a DRE, which is a fundamental test.
How is blood taken for the PSA test?
PSA analysis from a blood sample is performed in a laboratory using a technique called “radioimmunoassay”.
For this test, the patient must do the following:
- Not eat for at least 4 hours prior to the test;
- Not have sex in the 48–72 hours beforehand;
- Not do any exercise that strains the perineum (such as riding a bike, motorbike or horse, or going for long car drives) in the same period;
- Delay the test if you have had a recent urinary infection, prostatitis or urethritis, been catheterised, undergone a cystoscopy or had any other manipulation of the urinary tract.
Analysis of the PSA Test and Prostate Cancer
Prostate cancer generally causes a gradual, continual and moderately fast rise in PSA levels. Although not 100% reliable, the test is the best marker for cancer currently available.
Given this scenario, other methods have been developed to improve the sensitivity and specificity of the test (i.e., to reduce the rates of false negatives and positives).
Parameters can therefore be set such as:
- Ratio between free PSA and total PSA;
- PSA density;
- PSA velocity;
- Transition zone PSA;
- PSA as a function of age.
Ratio between Free PSA and Total PSA
PSA partly circulates freely in the blood and partly linked to proteins.
The former is called free PSA (fPSA) and the latter complexed PSA (cPSA) and their sum corresponds to total PSA (tPSA), which is the score obtained during routine PSA analysis.
The ratio between free PSA and total PSA provides more information in relation to the greater or lesser risk of cancer.
The lower the ratio, the greater the risk of cancer. A ratio/ split below 15% (0.15) is a clearer indication of cancer than a ratio higher than 20 or 25% (0.20 or 0.25).
The use of another score (called a cut-off or threshold score) has to do with the sensitivity and specificity chosen, i.e., the chosen risk not to diagnose a cancer vs the risk of doing an unnecessary biopsy.
To determine PSA density, the result of the test is related to the size of the prostate. The higher the density, the higher the suspicion of a prostate cancer.
To determine PSA velocity, repeated dosages of this test are assessed and compared to determine the velocity and rate of growth of the PSA score.
Up to a certain point, this growth is acceptable and above certain limits (the score generally referred to is 0.75ng/ml/year) the suspicion of a cancer is higher.
However, these scores have been questioned in some recent studies where the authors assert that even with lower increases, a malignant cancer could be present.
Transition Zone PSA
This is similar to PSA density but is only of consideration when a particular part of the prostate is of a certain size.
Therefore, the PSA score relates to the size of the transition zone.
PSA as a Function of Age
In this assessment, the PSA score is adapted to the age of the patient (given that PSA is known to increase with age, as well as the size of the prostate).
A PSA score of 2.5ng/ml, for example, may be acceptable at the age of 70 but unacceptable at 40.
Dr. José Santos Dias explains some of the myths and facts about the PSA test.
Dr. José Santos Dias
Clinical Director of the Instituto da Próstata
- Bacherlor's Degree from the Faculty of Medicine at the University of Lisbon
- Specialist in Urology
- Fellow of the European Board of Urology
- Autor dos livros "Tudo o que sempre quis saber Sobre Próstata", "Urologia fundamental na Prática Clínica", "Urologia em 10 minutos","Casos Clínicos de Urologia" e "Protocolos de Urgência em Urologia"
FAQs about the PSA Test
What is the PSA Test?
What are the benefits of doing a PSA test?
Are there any recommendations prior to doing the test?
How is PSA measured?
What indicates a high level of PSA?
- DIAS, José Santos. Book: Urologia Fundamental: na prática clínica. Lisbon: Lidel - Edições Técnicas, Lda, 2010.
- DIAS, José Santos. Book: Tudo o que sempre quis saber sobre a Próstata. Lisbon: Lidel - Edições Técnicas, Lda, 2014.
- Prostate-Specific Antigen (PSA) Test
- The PSA test