A comparison of fecal occult-blood tests for colorectal-cancer screening

Authors: Allison J, Tekawa I, Ransom L, Adrain A.
Source: New England Journal of Medicine. 334:155-9. January 18, 1996.
Institution: Kaiser Permanente Medical Center.
Financial support: Kaiser Foundation; SmithKline Diagnostics.

Summary

Background

Traditional guaiac-based fecal occult blood tests are relatively insensitive for the detection of adenomas and cancer. Rehydrated stool guaiac cards are more sensitive and have been shown to decrease mortality from colon cancer, but they are less specific and lead to many false positives. Newer approaches to FOB testing include more sensitive guaiac cards (such as Hemoccult II Sensa), and an immunochemical test for human hemoglobin (HemeSelect). This study was designed to compare three of these methods and a combination test.

Methods

Results Authors' Discussion

The authors feel that the combination test (Hemoccult II Sensa and, it this test is positive, confirmatory HemeSelect testing) represents the best approach. They assume that, as in their study, a positive Hemoccult II Sensa with a negative HemeSelect result will be followed by flexible sigmoidoscopy and Hemoccult II testing at 6 and 12 months. Although HemeSelect alone has a higher sensitivity than the combination test, it has a lower specificity and a much higher cost ($3.31 for Hemoccult II; $3.82 for Hemoccult II Sensa and $19.23 for HemeSelect). Since about 2/3 of the combination tests would not require HemeSelect testing, the cost of the combination test would be significantly less than HemeSelect testing alone.

Comment

In this study, the authors test each of three different types of fecal occult blood tests. The conventional guaiac test (Hemoccult II, here), has good specificity, but low sensitivity (around 30%), which would entail missing about 2/3 of cancers and polyps. The more sensitive guaiac tests, such as rehydrated stool guaiac tests and the Hemoccult II Sensa tested here have a much higher sensitivity, but their lower specificity would lead to a high colonoscopy rate. Finally, the immunochemical method, which detects human hemoglobin, has good sensitivity and specificity, but is very expensive. The combination test combines the advantages of the sensitive guaiac tests with the specificity of HemeSelect -- those who test negative by the sensitive guaiac test do not need the expensive immunochemical test; those who test positive by the cheaper test should be confirmed by immunochemistry.

In an accompanying editorial, D. Ransohoff and C. Lang state "Because neoplasms bleed intermittently, however, the next step in the evaluation of the combination test is to show that the sensitivity remains high when the test is applied to sequential specimens obtained at two different times, as Allison et al. suggest, instead of to the same specimens, as in their study." This is not my understanding of the combination test, however, which is not clearly defined in the article. My interpretation of the combination test is that all patients would provide simultaneous samples for both tests, and that the immunochemical test would only be performed if the guaiac test turned out to be positive. This is a very important distinction. Having a patient obtain three stool samples, followed by three more samples for the 1/3 of patients with a positive guaiac test would not be acceptable or practical.

The authors recommend performing sigmoidoscopy and repeat stool testing at 6 and 12 months for those with positive guaiac but negative immunochemistry tests. Unfortunately, they do not give data on the incidence of polyps and tumors in these patients, compared to those with negative guaiac tests. If these incidences turned out to be the same, even the flex sig and repeat stool testing might not be necessary, although it is hard to argue against them.

Assuming that the combination test is made available as a single, multi-specimen card, it certainly would seem to represent an improved fecal occult blood test approach.

2/27/96


Reader comments

Subject: Early detection of colorectal ca
Date: Thu, 25 Apr 1996
From: DSBB78A@prodigy.com (DR RICHARD L STERN)

 Remember that flexsig only studies about half of the colon. Aircontrast barium enema or a good single contrast barium enema is necessary, because about 30% of the significant polyps or cancers would otherwise be missed.
 



June 15, 1996

Letters to the Editor about this article from the NEJM

The question of the proper interpretation of the combination test, which I raised in my comments about this article, were addressed and answered here.


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