RECOMMENDATIONS FOR
ORDERING SERUM TUMOR MARKER TESTS

• Do not rely on the result of a single test. Because of the problem of nonspecificity associated with most tumor markers, it is difficult to distinguish between malignant disease or between benign and malignant diseases on the basis of a single test result. Serial testing can help detect falsely elevated levels due to transient elevation.
• When ordering serial testing, be certain to order every test from the same laboratory using the same assay. Different commercial assay kits may generate different results for the same analyte. It is important to exclude laboratory variability as a contributing factor to fluctuations in tumor marker levels.
• Be certain that the tumor marker selected for monitoring recurrence was elevated in the patient prior to surgery. Since none of the serum tumor markers are 100% sensitive to the detection of any particular cancer, it is important to be certain that the tumor marker ordered to detect recurrence was elevated prior to surgery. Multiple markers might be measured prior to surgery in order to select the optimal marker to monitor disease activity.
• Consider the half-life of the serum tumor marker when interpreting the test result particularly in the post-operative assessment of the success of surgical removal of tumor. It is important to wait an appropriate length of time after surgery because of the time required for the pre-existing tumor marker to decline to lower levels. For example, the half-life of serum PSA is approximately 3 to 4 days. It would take 30 days for a pre-surgical serum PSA level of 50 ng/ml to drop to an undetectable range following successful surgery.
• Consider how the serum tumor marker is removed or metabolized from the blood circulation. Serum tumor markers are frequently elevated in patients with underlying renal or liver disease.
• Consider ordering multiple markers to improve both the sensitivity and the specificity for diagnosis. The selection of multiple complementary serum tumor markers may be required to achieve optimal sensitivity when used as an adjunct in the diagnosis of malignancy. This reflects the heterogeneity of the tumor cells in many tumors as a result of multiple mutations in the development of a particular tumor.
• Order the nonspecific markers for cost-savings and for their high sensitivity. If the only objective is to monitor the efficacy of treatment, the use of nonspecific tumor markers may be considered. Although these nonspecific markers lack specificity for diagnosis and for relating to any specific type of tumor, their concentrations are very sensitive to any changes in tumor activity. Many of them are inexpensive and simple to measure and are therefore useful for monitoring therapy and detecting recurrence for patients with a known diagnosis.
• Be aware of the possibility of substrate exhaustion for certain assays. This phenomenon may result in a falsely low value when the tumor marker concentration in the specimen exceeds the limits of the assay. If a reported result seems incompatible with the clinical circumstances, then repeat testing using a diluted sample may be indicated.

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