PSA Index (PSA Ratio Free to Bound)

Immunology

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Description

The main application of all tumour markers is monitoring for relapse or progression. Use as a diagnostic aid requires care and knowledge of the test limitations. Opportunistic screening is discouraged. The major source of prostate specific antigen ( PSA ) is the ductal and acinar epithelium of the prostate, from where it is secreted into the seminal plasma. PSA is a zymogen of a 33 kDa serine protease with extensive homology with the glandular kallikreins. The predominant molecular form present in serum is the 80-90 kDa complex of PSA with a1-antichymotrypsin ( ACT ), free PSA represents a relatively minor but variable fraction of the total serum PSA concentration [2]. PSA is the tumour marker of choice for prostate cancer. It demonstrates high tissue specificity and serum levels correlate well with tumour mass and clinical stage [2]. NICE guidance for the diagnosis and treatment of prostate cancer recommend a combination of PSA measurement and rectal examination, with ultrasonography when investigating the condition [1]. Use of percent free PSA further improves the specificity of PSA testing, particularly in the range of 4-10 ug/L, at which most false positive PSA tests occur [3]. Current evidence suggests that the proportion of free and bound PSA differs between benign and malignant prostatic disease. The PSA index - free PSA expressed as a percentage of the total PSA - is usually below 12% in prostate cancer and above 12% in benign hypertrophy. Experimental data would suggest that up to 30% of prostate biopsies would be avoided if the PSA index was calculated in all those patients with a total PSA between 4 and 10ug/L [4].

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Indications

Prostate malignancy screening where PSA level is between 4 and 10 ug/L.

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Sample Type

2mL Serum ( Gel 5mL Yellow tube ). Requests from outside Sheffield: Transport at ambient temperature via Royal Mail or Courier.

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Reference Range

Greater than 12% ( Refers to method used in this laboratory only ).Reference range established in house.

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Turnaround Time

Within 2 days

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Testing Frequency

Daily

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References

NICE Clinical guideline 58 - Prostate cancer diagnosis and treatment. 2008 [Ref 1]PRU Handbook of Clinical Immunochemistry. 9th Edition. 2007. [Ref 2]Canto EI, and Slawin KM. Early management of prostate cancer: how to respond to an elevated PSA? Annu Rev Med. 2002. 53:355-368. [Ref 3]Martinez-Pineiro L, et al. Determination of the percentage of free prostate-specific antigen helps to avoid unnecessary biopsies in men with normal rectal examinations and total prostate-specific antigen of 4-10 ng/ml. Eur Urol. 2000. 37( 3 ):289-296. [Ref 4]

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