Skip navigation

Kit Watch Serum PSA

Serum PSA predicts negative bone scan

The commonest metastatic site in prostatic cancer is bone, and to that end many patients with newly-diagnosed prostate cancer undergo bone scans following administration of technetium-methylene diphosphonate. These are expensive, and are not without discomfort to patients. Two studies from the Mayo clinic in recent years have indicated that a simple blood test can accurately predict a negative bone scan.

Initial study shows 99.7% prediction

A retrospective study in 1991 of 521 randomly chosen patients with newly-diagnosed untreated prostate cancer examined local clinical stage, tumour grade, enzymic serum acid phosphatase and immunoreactive PAP as well as PSA (prostate specific antigen). All were related to the results of bone scans.

The authors used relative operating characteristics (ROC) analysis to examine the power of the predictive value of each test, alone and in combination, in predicting the results of the bone scan. PSA was far the best: median PSA value in patients with a positive scan was 158 µg/L compared with 11.3 µg/L in those with a negative scan. Using multivariate logistic regression analysis, no combination was better than PSA alone.

In 306 men with a PSA of 20 µg/L or less, only one (PSA 18 µg/L) had a positive bone scan. The negative predictive value was 99.7% (95% confidence interval from 98.2 to 99.9%).

Confirmation of original findings

In a follow-up study in 1993 the records of 2064 consecutive patients with prostate cancer (calendar years 1989 and 1990) were evaluated, and 852 patients with newly-diagnosed, untreated disease and a serum PSA of 20 µg/L at presentation were chosen. The main outcome measure was the rate of false-negative results associated with using serum PSA to predict negative bone scan findings. The raw results are shown in the Table. 98.8% of patients had negative bone scans. Seven had abnormal scans, but five of these had pain symptoms suggestive of metastatic bone involvement. Excluding these, who would have been examined because of the clinical findings, the predictive rate was 94.4%. No patient with an abnormal scan had a PSA of less than 8 µg/L.

[Data from Mayo Clinic, 1993]

At a variety of cut-off levels between 4 and 20 µg/L, the 95% confidence limits of false-negative scans were at worse 0.3 to 1.7%. An asymptomatic patient with untreated prostate cancer with a serum PSA of 10 µg/L or less has a 0.5% likelihood of having an abnormal bone scan. The scan thus provides no additional information.

Cost implications

In the USA, where there are 132,000 new cases of prostate cancer diagnosed each year (similar to EC countries). About 39% will have PSA values of 10 µg/L or less, meaning that at least 52,000 unnecessary scans are performed, at an average cost of $600. Potential savings in the USA amount to $30 million a year. In 1987 there were 11,000 new case of prostate cancer in England & Wales. Pro rata savings for the NHS would be £2 million a year. The increasing incidence of prostate cancer because of ageing populations, and easier, better and earlier diagnosis, make this likely to be an underestimate.

References:

Chybowski et al. Journal of Urology 1991 145: 313-8.
Oesterling et al. Journal of the American Medical Association 1993 269: 57-60.

Questions to be Answered

Q: What need is met by this test?
A: Prediction of negative bone scan and eliminating the need for that procedure.
Q: What happens at present?
A: All newly-diagnosed patients with prostate cancer have a diagnostic bone scan.
Q: How does the test improve quality?
A: It provides an answer without invasive procedures.
Q: What is the cost?
A: Most biochemistry labs could provide the test at a cost of about £4-6 (reagents and labour) for every case of prostate cancer. PSA kits are readily available and there is no capital cost. A PSA service should be available.
Q: What cost savings are likely?
A: There should be a saving of about £400 per case in about 40% of cases of newly diagnosed prostate cancer. The overall saving should be about £150 per case of newly diagnosed prostate cancer.

Advice to Purchasers

  • Will increase quality and effectiveness.
  • Will decrease total cost of care.
  • Should be included in specification.

previous story in this issue