Prostate Specific Antigen (PSA), Free (Unbound)
Label Mnemonic: FPSA
Epic Lab Code: LAB117
Downtime Form: A-1a Doctor/Provider Orders - Pathology Core and Specialty Care Nursery
6240 RCP
Collection Medium:
Plasma Separator Tube 4.5 mL
Alternate Collection Media:
Call laboratory for additional acceptable specimen collection containers.
3 mL whole blood in light green top tube or TWO Microtainer® devices.
Testing Schedule:
24 hrs/day, 7 days a week, including holidays.
Turn Around Time:
1 hour (upon receipt in laboratory)
Reference Range:
No accepted reference range. There is an increased probability of prostate cancer with lower %FPSA.
This assay may be significantly impacted by high-dose biotin (>5 mg dose) taken within previous 12 hours. High concentrations of biotin may lead to falsely decreased results. These concentrations may be found in patients taking over-the-counter supplements with biotin content much higher than nutritional requirements for biotin. Specimens should not be collected until at least 12 hours after the last dose.

Prostate-specific antigen (PSA) is a glycoprotein (molecular weight 30,000-34,000 daltons) having a close structural relationship to glandular kallikrein. It has the function of a serine protease.

The proteolytic activity of PSA in blood is inhibited by the irreversible formation of complexes with proteinase inhibitors such as alpha-1-antichymotrypsin, alpha-2-macroglobulin and other acute phase proteins. In addition to being present in these complexes, PSA is also present in blood in the free form, but is proteolytically inactive.

PSA tests lack sufficient sensitivity and specificity to be considered ideal or absolutely diagnostic for screening or early detection because PSA is not specific for prostate cancer. PSA is organ specific, being produced primarily by prostatic secretory epithelium, but has long been known to be elevated in non-malignant conditions such as benign prostatic hyperplasia (BPH). A number of studies have found that the % free PSA was significantly lower in patients having prostate cancer than those with benign disease or normal controls. The ratio fPSA/tPSA has been demonstrated to improve the sensitivity and specificity in patients with tPSA values in the "gray zone" of 4-10 ng/mL.

An equimolar tPSA determination is the prerequisite for reliable ratios.

In patients receiving therapy, particularly hormone withdrawal therapy, the fPSA/tPSA ratio cannot be utilized to differentiate prostate hyperplasia from cancer of the prostate. Combining tests from different manufacturers to determine tPSA and fPSA can produce erroneous values, since total PSA tests may be standardized by differing methods or detect free PSA to differing degrees.

The free PSA immunoassay is indicated for measurement of fPSA in conjunction with the total PSA assay to develop a ratio of fPSA to tPSA (%fPSA). This ratio is useful when used in conjunction with the Total PSA test as an aid in distinguishing prostate cancer from benign prostatic conditions in men age 50 years or older who have a digital rectal examination (DRE) that is not suspicious for prostate cancer and an total PSA value in the range 4-10 ng/mL. Prostate biopsy is required for the diagnosis of prostate cancer.

This test is also performed in the Iowa River Landing (IRL) clinical laboratory (for specimens drawn at that site).
Test Limitations:
The assay is unaffected by indexes of icterus <65 mg/dL, hemolysis <1000, lipemia <1500 mg/dL and biotin < 30 ng/mL. In vitro tests were performed on 28 commonly used pharmaceuticals. Only flutamide at therapeutic daily dosage levels resulted in slightly depressed free PSA values. As with all tests containing monoclonal mouse antibodies, erroneous findings may be obtained from samples taken from patients who have been treated with monoclonal mouse antibodies or have received them for diagnostic purposes. Free PSA contains additives which minimize these effects. In rare cases, interference due to extremely high titers of antibodies to streptavidin can occur. There is no high-dose hook effect for free PSA concentrations up to 15,000 ng/mL. No influence was observed from rheumatoid factor (up to 1500 U/mL). For diagnostic purposes, the free PSA findings should always be assessed in conjunction with the patient's medical history, clinical examination and other findings.
Electrochemiluminescence Immunoassay
CPT Code:
See Additional Information:
Biotin Interference with Immunoassays