Medicine For People!

October 2010 Appendix

Risk Model

Up until now, medical opinion leaders have ignored the affordability question. Finally, as the system crashes down around us, pragmatism returns. We need to target our resources to those who are at risk. The illustration below gives a statistical model for evaluating risk

Copyright © 2010 Massachusetts Medical Society. All rights reserved. "Lessons from the mammography wars" by Quanstrum KH, Hayward RA. N Engl J Med. 2010 Sep 9;363(11):1076-9 Used by permission.

In the case of colon cancer screening, people over a certain age would not benefit. People with a genetic make-up that almost guarantees colon cancer would undergo colonoscopy without further question. And those in the middle zone, most of us, would be treated as individuals.

How the iFOBT works

To perform the iFOBT test, the laboratory measures the concentration of hemoglobin in the stool. As you can see in the following graph, almost all people with no polyps or with benign polyps score less than 75 nanograms hemoglobin per milliliter, and the test in reported as negative.

Number of people with normal colon or minor polyps

Hemoglobin in stool, nanograms/milliliter

Source of all graphs on this page: Ann Intern Med. 2007;146:244-255.

(Looking at this diagram, the height of the bars represents the number of people with that result, so that the left hand bars, sitting on the 0 on the x-axis, show that most people with no polyp, or a normal polyp, show no hemoglobin in the stool. The lower bars further to the right show that a few of such people have small amounts of hemoglobin in the stool, but the great majority measure under 75 nanograms/milliliter (ng/ml) of stool.)

Now look at the same test in people who had cancer.

Number of people with colon cancer, and amount of hemoglobin in stool

Hemoglobin in stool, nanograms/milliliter

Most score above the 75 ng/ml cutoff, the result at which the test is reported as "positive."

When we look at people with advanced polyps, we see varying results.

Number of people with polyps over 2/5 inch diameter or abnormal but non-cancer cell pattern

Hemoglobin in stool, nanograms/milliliter

As the polyp progresses, it produces more blood, which is why an annual test provides the best protection, detecting a polyp about the time it becomes of concern.

In other words, the test is not positive for people with a normal colon or with non-malignant polyps, and can separate early cancer from normal colon with a reasonable degree of probability. As polyps progress, as we outlined two months ago, the more likely the iFOBT is to turn positive.

story: 

Medicine for People! is published by Douwe Rienstra, MD at Port Townsend, Washington.