A fecal immunochemical test for hemoglobin is a test designed to detect hemoglobin in feces using an antibody raised against human hemoglobin. It is one of a class of technologies referred to as fecal occult blood test (FOBT).
Fecal immunochemical tests (FIT) incorporate an antibody specific for human hemoglobin; such react with the globin protein moiety of hemoglobin.
Fate of Hemoglobin in the Gastrointestinal Tract
The globin moiety of hemoglobin is a protein and as such is rapidly digested in stomach or small intestine such that it is no longer detectable by immunochemical methods. Ingestion of blood volumes of up to 100 mL has not resulted in detectable immunoreactive hemoglobin in feces, based on reports using some of the earlier types of FIT. In other words, FIT are selective for colorectal bleeding, especially where the bleeding is occult (i.e., not visible to the naked eye). This is different from the chemical, specifically guaiac-based, FOBT which can return a positive result for blood derived from the proximal as well as distal GI tract.
Because fecal immunochemical test technology is so different from that of the chemical methods, because it reacts to a different analyte (i.e., globin not heme) and because it provides the option of quantification of fecal hemoglobin rather than being limited to qualitative assessment as is the case for GFOBT, it is recommended that the terminology “fecal immunochemical test” be applied and abbreviated as FIT, to emphasize that it is a quite different technology and analyte from chemical FOBT.
Uses for FIT
The primary use of FIT is in screening for colorectal cancer. Chemical FOBT have been proven to reduce mortality and incidence in four randomized controlled trials. Comparative studies have shown FIT to be more sensitive for cancer and more sensitive for colorectal adenomas than guaiac-based FOBT. A hemagglutination-FIT has been shown to be superior to a high-sensitivity guaiac-FOBT in that it achieved the same higher sensitivity for cancer but required less than half as many colonoscopies to achieve this. In other words, FIT improves sensitivity for colorectal cancer and adenomas but not at the cost of unacceptable deterioration in specificity.
FITs are not subject to interference by diet or drugs and maintenance of an adequate specificity is not dependent on proscription of certain foods or drugs as is the case for guaiac-FOBT. This, together with the improved stool-sampling methods characteristic of FIT, makes it easier for screenees and improves participation rates in screening.
It is not advisable to use FIT as an all-purpose test for fecal blood as it is selective for colorectal bleeding. If used in this setting, such as in the context of iron-deficiency, it should be combined with a chemical test. However, it is important to note that the negative predictive value of either guaiac-FOBT or FIT is not 100% and many consider their use should be confined to screening for colorectal cancer where they act as an aid to early detection; as such they serve to identify those most likely to have colorectal neoplasia and so in need of a diagnostic procedure such as colonoscopy.
Several of the latest FIT allow quantification of fecal hemoglobin. Results show that fecal hemoglobin concentration corresponds to the “stage” of neoplasia. People with cancer have the highest levels on average, those with a normal colon the lowest levels while those with adenomas have intermediate levels. Studies such as these show that patients with advanced adenomas (size greater than 1 cm, villous change or high-grade dysplasia) do have occult bleeding. In other words, the higher the level of fecal hemoglobin, the more likely that neoplasia is present.
The advantage of fecal hemoglobin measurement is that it returns full control of sensitivity and specificity to the doctor managing screening. It enables the doctor to set the level of fecal hemoglobin that would trigger follow-up diagnostic colonoscopy. At the population level, such an approach allows flexible choice of a test specificity:sensitivity ratio, so controlling the colonoscopy rate to that which is feasible for a health care system.
Types of FIT Currently Available
A variety of test technologies are employed in the currently available tests. This includes latex agglutination, membrane diffusion devices for detecting immunogold- or latex-labeled antibodies, hemagglutination, and magnetic particle agglutination. The end point for some tests is read by eye while others may also be read by a machine designed specifically for the particular technology and its end point. Some devices are designed for simplicity and development in an office setting while others are designed for large-scale development in a laboratory – automated processing, development, and reading may be available.
Internationally, there are no set criteria that a test manufacturer should follow when setting the specifications for a test. As such, different tests may behave differently with differences in the quantity of hemoglobin to which they react as well as the nature of the hemoglobin antigen that they detect.
A test result may be quantitative, providing a measure of the concentration of hemoglobin in feces, or qualitative, providing an indication as to whether the fecal hemoglobin is considered to have reached a level likely to reflect the presence of neoplastic pathology in the colon or rectum.
The nature of the fecal sample is an important component of a test. Each test comes with its own characteristic stool-sampling device. These include devices that keep the sample dry and so more likely to keep the antigen stable and others that use devices that include liquid and in which hemoglobin degradation might proceed during specimen transit unless samples are kept cold.
Most devices require the stool to be kept clear of toilet bowl water when sampling while a few require sampling to be undertaken from a stool kept clear of water. The range of sampling devices include wooden spatulas that smear feces onto a card, probes that absorb or wick materials from feces when poked into the stool, probes that trap feces into grooves when poked into the stool, and brushes that sample toilet bowl water from the surface of the immersed stool. Some provide precise control of stool sample quantity while others do not provide such control.
Most manufacturers provide packaging that enables samples to be sent by mail to a processing laboratory.
As hemoglobin can be rapidly degraded in feces especially at room temperature or above, whole stools should never be sent to a laboratory for sampling by the laboratory. Rather, the appropriate stool-sampling device provided for a test must be used. It is likely that different tests vary in their capacity to detect partially degraded hemoglobin. This will be dependent on the antibody used but there is little published material to guide users.
Performing an in-office GFOBT as part of a digital rectal examination has been common practice especially in the USA. However, usually only one rather than the recommended two or three stool samples is obtained. Recently this approach has been evaluated in a large group of asymptomatic subjects where it was found to be unsatisfactory. Screening based on digital sampling of stool as part of a rectal examination cannot be recommended and manufacturer’s recommendations for obtaining stool samples should be followed.
Biology of Bleeding from Colorectal Neoplasia
The fact that microscopic bleeding may arise from cancers or adenomas provides the basis for the use of FOBT to aid early detection of colorectal neoplasia. Blood loss is variable from day to day and may fall within the normal range. Furthermore, blood of colorectal origin is not uniformly distributed within a stool and often is found on the surface. Consequently, it is usually recommended that when screening for colorectal cancer, samples from at least two separate stools be collected. It is also recommended that the stool surface be sampled rather than the stool interior or a homogenate.