The challenges of prostate cancer detection

Medical Laboratory Observer, Feb, 2004 by Carren Bersch

The American Cancer Society estimates that 220,900 men were diagnosed with prostate cancer last year in the United States, with 28,900 deaths attributable to the disease. When the prostate biopsy test results for one of our co-workers came back with a positive result for cancer in 2002, we learned that statistics, however shocking, are fairly impersonal until one of those 220,900 is an acquaintance, a friend, a relative, or a business associate.

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In 2001, our co-worker tested negative for prostate cancer. A year later, the same test was performed and four of his 11 samples were cancerous. In both years, the specimens were sent to a lab owned by the urology team that did the biopsy. Subsequent tests on the 2002 samples by two other laboratories showed that the cancer was present, yet these laboratories had different results than the first. One of the outside labs indicated that the cancer was more aggressive (a Gleason 8 score) than the other labs that showed a maximum Gleason score of 7. The Gleason 8 score required more radiation and two years of medication, whereas a Gleason 7--had it been the accurate test--would have required less radiation and only three months of medication.

Many questions were asked during the lengthy process of our associate's recovery. How variable are test results from one laboratory to another? How can laboratorians get accurate results the first time? How do different methods and commercial assays for the same disease obtain widely differing results? Would improved standards affect testing? For insight into these and other questions, we approached Richard J. Zarbo, MD, DMD, and John E. Tomaszewski, MD, for some answers.

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Sensitivity and specificity

John E. Tomaszewski, MD: In reference to the submitted case, three related but separate issues with respect to problems in laboratory testing for prostate cancer are illustrated. The first issue has to do with the localization of prostate cancer within a gland. The second is the microanatomic recognition of carcinoma in tissue section. The third is the use and development of prognostic markers in prostate cancer.

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Richard J. Zarbo, MD, DMD: The entire area of prostate cancer detection is problematic for patients and doctors alike, as there are many factors that may impart variability in serologic PSA and biopsy test results. Much of the anxiety for men is that the PSA test is not a specific marker of prostate cancer and is, therefore, not a good cancer-screening test. However, at this time it is all we have. Serum PSA levels will rise not only in prostate cancer but also in noncancer associated states--with increasing age as the gland enlarges under the influence of testosterone (benign prostatic hyperplasia), with infection and inflammation of the gland (prostatitis), with urinary tract infection, and post-ejaculation. Yet, some aggressive prostate cancers, especially in young African-Americans, may present with only slightly elevated PSA levels. To improve this poor test sensitivity and specificity, other PSA parameters may be evaluated, such as age-adjusted cut-offs, PSA density determined by dividing the PSA level by the volume of the gland determined by TRUS, PSA velocity which tracks the PSA increase on three specimens over time where velocity over 0.75 ng/mL is high, and free PSA which is the amount of circulating PSA unbound to blood proteins where a low percent of free PSA is worrisome for cancer.

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Pre- and post-analytic variance

Tomaszewski: The standard PSA test is an automated assay with little in the way of analytic variance. While it is always possible for an individual lab technician to make an error in the operation of the analyzer, the standard management of clinical laboratories makes that very unlikely. A much more significant problem is with pre-analytic variance. This refers to all the problems, which can happen before the test even gets to the machine. For example, if the specimen is mislabeled or poorly handled, a value may be in error. Clinicians need to take more responsibility in assuring the lab that the materials (blood, urine, tissue) they submit for testing are properly labeled, handled, and transported. Point-of-care testing is coming, which will move instrumentation to clinicians' offices and likely introduce more variance in the analytic component of the testing cycle, since nonlaboratory professionals will operate this instrumentation. This is a patient-safety issue that should be watched very carefully in the future.

If a lab test is modified, it becomes a new test; this new test has a new reference range and new operating characteristics. This means that the sensitivity, specificity of the test in relation to the disease prevalence of interest has to be re-evaluated for the new test. Labs are responsible for publishing the appropriate new reference ranges. The clinicians are responsible for integrating these reference ranges into their recognition of the value of the test for a particular patient. For example, age and race stratification are critical pieces of information that factor into the interpretation of a PSA value. Prostate size and interval changes are other important features. These factors can only be meaningfully integrated into some decision analysis by the clinician. Unfortunately, many primary care clinicians are much uninformed regarding these matters. This variability of interpretive acumen is termed post-analytic variance. The most effective improvements in PSA testing can be made in addressing the places where there is the most variance (i.e. the pre- and post-analytic areas).