Use of faecal occult blood tests in symptomatic patients

I welcome the current discussion about the use of tests for detecting occult blood in stool in symptomatic patients (1-3). The uncertainty about what to do is illustrated by the different practice in countries as similar as Denmark and Norway. In Denmark, GPs are not encouraged to use the test, mainly based on the low sensitivity argument also forwarded by Steele et al. Danish guidelines recommend FOBT use in secondary care after a negative sigmoidoscopy (4). In Norway and Sweden, GPs frequently use the FOBT test.

Old articles being hard to find, I would like to refer to my article in “Allgemeinmedizin” in 1990 (5), where positive predictive value for colorectal cancer in general practice patients with “indigestion” increased considerably when a FOBT was positive, and more so in higher age groups, PPV depending on prevalence. Like the six research papers identified in the NICE update, I think my findings defend the use of FOBT in symptomatic patients.

How the GP works
The low sensitivity argument is important, and I can understand that the possibility of a false negative test is particularly worrying in patients with anemia (2). Patients with anemia do not necessarily have a positive FOBT, and vice versa (6). However, all GPs should know that anemia discovered in general practice always –always- needs to be explained. That implies that further testing or referral should follow initial negative tests until the reason for anemia has been clarified. One negative test, FOBT or other, was never meant to be conclusive about non-disease in general practice. A GP usually needs to rely on the patient collection of broad information through a good medical history, appropriate clinical examination and selected laboratory tests and imaging (7), often using a follow-up consultation but without wasting time.

Specificity and cut-off
The specificity has been given less attention in the debate, but is important. The specificity of FOBS tests is clearly too low for population screening purposes, although such practice exists. The use in symptomatic patients is different. A high specificity is important to avoid unnecessary investigations in healthy people, but the number of false positives is limited when applied to symptomatic patients. FOBTs are designed to detect a minimum concentration of blood in stool. We all shed a minimal amount of red blood cells from the intestine every day, and the cut-off chosen for tests must balance the need for high sensitivity against the need for high specificity. If one goes up, the other goes down. Fecal immunochemical tests (FIT) probably have advantages over guaiac-based tests (gFOBTs) (8), but FIT are till now in little use in general practice and lack documentation from this setting.

Sensitivity
The sensitivity for gFOBTs is probably higher that the 50% figure cited by Steele et al, who cites Young et al who cites an article (9) with a research setting removed from real life situations for GPs. The most-used gFOBT in Norway and several other countries, Hemo-fec®, in a doctoral thesis from 1984, was found to have a sensitivity for colorectal cancer of 85% and for colorectal polyps 50% (10), and for gastric cancer 85% (11). The thesis also showed that in spite of great variation, cancers both in colon and stomach tend to bleed more than benign lesions. There were 20 cases of colorectal and 23 cases of stomach cancer in this study. In spite of the high sensitivity figure for gastric cancer in this study, bleeding from the lower gastrointestinal tract was more easily detected that from the upper part (10). It is possible that the difficulty of diagnosing cancers of the ascending colon in part is due to a similar effect that adds to the paucity of early symptoms in this part of the colon. If FOBT sensitivity is lower for cancers in the ascending part, the Danish guidelines may increase delay. I think FOBT should be performed in general practice in accordance with the revised NICE guidelines rather than after a negative sigmoidoscopy. Capacity considerations may have different consequences, but in general, a positive FOBT should lead to colonoscopy. If negative, a gastroscopy, an ultrasound of abdomen or a CT scan are possibilities to be considered in the individual case.

Understanding FOBT
No tests are good unless well understood and appropriately performed. Cancers bleed intermittently. Pairs of specimen should therefore be taken on three different days, and one positive test is enough to consider the test positive. This may increase the number of false positives, but reduces the fear of false negatives. The patient must follow advice concerning medication and diet during 3-4 days prior to testing. The most important probably are avoidance of NSAID, acetyl acetic acid, vitamin C, red meat and raw vegetables. Anticoagulants make interpretation of a positive test difficult. Interpretation of FOBT applied to stool specimen is not difficult for experienced laboratory personnel.

Knut Holtedahl
GP, Professor emeritus, UiT The Arctic University of Norway, Tromsø, Norway
knut.holtedahl@uit.no

References
1. Hamilton W, Hajioff S, Graham J, Schmidt-Hansen M. Suspected cancer (part 2-adults): reference tables from updated NICE guidance. BMJ. 2015;350:h3044.
2. Steele R, Forgacs I, McCreanor G, Benton S, Machesney M, Rees C, et al. Use of faecal occult blood tests in symptomatic patients2015 2015-08-11 16:11:20.
3. Hamilton W, Hajioff S, Graham J, Schmidt-Hansen M. Authors’ reply to Steele and colleagues2015 2015-08-11 16:01:30.
4. Sundhedsstyrelsen. Pakkeforløb for kræft i tyk- og endetarm (Fast track examination for colorectal cancer). In Danish. Copenhagen 2012.
5. Holtedahl KA. Probability revision in general practice: the cases of occult blood in stool in patients with indigestion, and daily smoking in patients who cough. Allgemeinmedizin. 1990;19:35-8.
http://www2.uit.no/ikbViewer/Content/237422/Probability.pdf
6. Scheel BI, Holtedahl,K. Symptoms, signs and tests: The general practitioner's comprehensive approach towards a cancer diagnosis. Scand J Prim Health Care. 2015;In Press.
7. Holtedahl KA. Diagnosis of cancer in general practice. A study of delay problems and warning signals of cancer, with implications for public cancer information and for cancer diagnostic strategies in general practice. : University of Tromsø, Norway: ISM skriftserie nr. 16, 1991. http://hdl.handle.net/10037/2325
8. Young GP, Symonds EL, Allison JE, Cole SR, Fraser CG, Halloran SP, et al. Advances in Fecal Occult Blood Tests: the FIT revolution. Dig Dis Sci. 2015;60(3):609-22.
9. Lansdorp-Vogelaar I, van Ballegooijen M, Boer R, Zauber A, Habbema JDF. A novel hypothesis on the sensitivity of the fecal occult blood test: Results of a joint analysis of 3 randomized controlled trials. Cancer. 2009;115(11):2410-9.
10. Dybdahl J.H., Daae L.N.W., Larsen S., Myren J. Occult faecal blood loss determined by a 51Cr method and chemical tests in patients referred for colonoscopy. Scandinavian journal of gastroenterology. 1984;19:245-54.
11. Dybdahl JH. Occult faecal blood loss determined by a 51Cr method and chemical tests in patients referred for upper gastrointestinal endoscopy. Scandinavian journal of gastroenterology. 1984;19:235-44.