4

The patient used an EZ Detect fecal occult blood test. The test was positive. She saw a tiny blue circle on the piece of paper indicating trace of blood in her stool. But, she indicated there was also urine in the toilet bowl. Would urine affect this test?

  • Just curious, but didn't the directions that came with your hemoccult test give you all of this information? – anongoodnurse Jan 31 '16 at 9:04
  • No, it did not. – Sympa Jan 31 '16 at 23:16
  • "Step 2: Urinate first and then flush the toilet; have a bowel movement; carefully drop a Test Tissue into the toilet bowl..." From instructions. – anongoodnurse Feb 4 '16 at 22:38
-1

After doing some research on the issue, including contacting the company "Biomerica" and finding on the Internet the "Physician's Instruction Pamphlet" I did find that urine does affect such tests. The mentioned pamphlet states: 1) Urinate first and then flush the toilet; 2) Have a bowel movement; 3) carefully drop a test tissue into the toilet bowl, etc. Thus, Biomerica is adament that for the test to be reasonably accurate urine should be absent from the toilet bowl. They also confirmed the same by email.

One should keep in mind that even if you take all those precautions you should understand that such tests are not "accurate" especially when they are positive. This is due to such tests having a rather low sensitivity (true positive rate) combined with a disease that has a fairly low prevalence rate. Quoting Wikipedia on fecal occult blood test with a positive result: "about 2-10% have cancer, while 20%-30% have adenomas." Depending on one's circumstances those statistics can vary (age, sex) and increase a bit. But, still a positive test result still leaves one much more likely to be free of colon cancer. It is just an information piece that may suggest further test investigation may be warranted (colonoscopy, etc.).

In view of the above, following Biomerica's instructions I would not consider a positive test result with urine in the bowl as counting as a true positive test. Instead, I would continue testing with the remaining test tissues and make sure I follow the appropriate protocol as indicated above. If one would feel they did not have enough remaining test tissues, I would just get another set and re-test again. If the individual does test negative three times in a row that should give her much comfort. The test is far more accurate on "negatives" vs. "positives" because the test's specificity (true negative rate) is much above its sensitivity (true positive rate).

  • such tests are not "accurate" especially when they are positive. This is due to such tests having a rather low sensitivity (true positive rate).... -- When faced with a positive test, specificity rather than sensitivity is relevant. "True positive rate" is ambiguous (if describing sensitivity it indicates the proportion of true positives that are correctly identified rather than the proportion of identified positives that are true), but in any case false positives are due to poor specificity. – Susan Mar 6 '16 at 8:40
  • The accuracy of a positive test is equal to: # true positive observations/(# true positive obs. + # false positive obs.). So, sensitivity (true positive rate) plays an important role in both numerator and denominator. Specificity (true negative rate) plays a role in generating # false positive observations. However, the big driver in generating # false positive obs. is not so much specificity but the prevalence rate of the specific disease being screened. A test with high specificity testing a disease with low prevalence rate will still generate a lot of false positives. – Sympa Mar 7 '16 at 5:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.