Unopened sample bottles may be stored at 1-30°C and can be used up to the use-by date stated on the label.
FIT immunochemically detect the globin component in the haemoglobin protein. For this reason, they have a significantly better analytical specificity for human haemoglobin but are sensitive to proteolytic degradation of the haemoglobin protein by microbial proteases in the gut flora. This degradation starts in the gut and continues after the sample has been taken.
Haemoglobin in native faeces (with no stabilising buffer) is highly unstable. Haemoglobin concentrations that had been determined in native faeces by means of various different FIT tests were reduced within a few days to below the detection limit.
The degradation of haemoglobin can be slowed significantly through the use of optimised, stabilising buffer systems, into which the stool sample is transferred by the user as soon as it has been taken. In the case of OC-Sensor™ FIT, a stabilising sample buffer of this type is integrated into the sample-taking system (OC-Sensor™ sample bottle).
The outstanding efficacy of the OC-Sensor™ sample buffer system has been confirmed in both internal and external trials.
Stool samples in OC-Sensor™ sample tubes may be stored, between being taken and their analysis in the laboratory, for one week at room temperature or 28 days under refrigeration.
The period of time in which the samples are exposed to varying temperatures should be kept to a minimum, including but not limited to, the transfer of samples between collection and analysis and the storage of samples at room temperature.
Manufacturer’s statement: For samples that were stored in OC-Sensor™ sample tubes for 28 days at 2-8o C, a haemoglobin detection rate of 95% ± 14.7 (2 SD) was calculated. After 7 days’ storage at 25oC a detection rate of 96% ± 20.4 (2 SD), while at 30oC 89% ± 20.5 (2S D), was calculated.
The screening test does not directly detect the targeted lesions but instead detects occult human blood in the stool. Thereby these tests provide an indirect indication of the existence of colorectal tumours or their precursors, since these bleed more frequently than healthy intestinal mucosa. The average quantity of haemoglobin detected increases with the progression of colorectal cancer, from a neoplasm with low risk via advanced neoplasms right up to colorectal carcinoma; a direct correlation could be proved between the haemoglobin content in the stool (FIT result) and the appearance of neoplasms. Admittedly the method does not permit any precise statement about the targeted lesion on the basis of the haemoglobin content ascertained.
The haemoglobin content is dependent on age and sex; the haemoglobin content is higher in men and older individuals. This was also able to be determined on the basis of data from three different countries (Taiwan, Scotland and Italy). The FIT data was ascertained using the same test system (OC-Sensor™) in all cases and they are therefore directly comparable. In all of the countries a significant correlation was seen between haemoglobin concentrations and age plus gender, however the extent of these differences varied among the countries. This argues for the use of different individual cut-off concentrations.
Furthermore quantitative FIT results in conjunction with other risk factors (age, gender, previous family loading, etc) offer an interesting new possibility for risk stratification and higher accuracy in FIT-based colorectal cancer screening. FIT concentrations are associated with the degree of the histological findings and may improve the predictability in the risk of colorectal neoplasms.
McDonald PJ et al.
Faecal Haemoglobin Concentrations by Gender and Age: Implications for Population-Based Screening for Colorectal Cancer
Clin Chem Lab Med 50/5 (S. 935-40); 2012
Fraser CG et al.
Faecal Haemoglobin Concentrations Vary with Sex and Age, but Data Are Not Transferable across Geography for Colorectal Cancer Screening
Clin Chem Lab Med 52/8 (S.1211-6); 2014
In general population screening -
In 2016 the UK National Screening Committee recommended a change to the test used in the NHS Bowel Cancer Screening Programme from the current guaiac faecal occult blood test (gFOBT) to FIT based on the improved sensitivity, specificity and great detection of cancer and advanced adenoma in general populations. FIT has now replaced gFOBT in screening programmes in Scotland, Wales and most recently England. Northern Ireland currently use gFOBT for screening.
In symptomatic patient testing -
A major challenge facing clinicians is to identify patients suspected of colorectal cancer in primary care. Abdominal symptoms presented by patients to their GP could be indicative of a number of diagnosis, including cancer, however currently >90% of colonoscopies carried out as part of referral pathways for early cancer detection are negative for cancers. In 2015 the National Institute for Health and Care Excellence (NICE) issued a revised NG12 'Suspected Cancer: Recognition and Referral' guideline, which now includes the use of faecal occult blood resting as part of the two week wait urgent referral pathway for symptomatic patient cohorts. In 2017 NICE Diagnostic Guidance 30 endorsed the use of FIT to triage low risk patients that do not meet the criteria for the NG12 pathway. Evidence shows that at low cut off levels, FIT has a very high negative predictive value for colorectal cancers. FIT in a symptomatic population can therefore be used to risk stratify patients and avoid unnecessary referrals for colonoscopies. NICE set a distinct cut off for symptomatic patient testing. This cut off value differs from the cut off used in general population screening testing.
FIT gFOBT Test Principle
Immunochemical Turbidimetry using antibodies specificto human haemoglobin
Oxidation of guaiac by a hydrogen peroxide developer to form a colour change in the presence of haem Test Target Globin specific to human haemoglobin Any Haem present Analysis Automated via analysers Manual Result Data Quantitative (recommended) or qualitative Qualitative only Interpretation Reaction turbidity read by objective analyser Colour change subjectively read by eye Interferences None recorded Red or rare meat, some vegetables, anti-inflammatory and iron supplement medication Dietary Requirements None Some foods should be avoided before testing Sample Collection Single stool sample is collected into dedicated device containing stabilising buffer Multiple stool samples, collected from different bowel movements, smeared onto a testing card Samples required 1 3 Repeated Analysis Yes No - can only be tested once Hb cut off flexibility Adjustable cut off
The haemoglobin concentration in the OC-Sensor™ FIT is determined in nanograms of haemoglobin per millilitre of buffer in the sample tube (ng/ml).
The pre-analytics and sample extraction are standardised for all OC-Sensor™ sample tubes:
Each sample tube contains 2 ml of stabilising sample buffer in which, with the aid of the test-wand, 10 mg of stool sample is suspended.
The haemoglobin concentration, determined by the OC-Sensor™ analysis device in ng per ml of sample buffer, is specific to the OC-Sensor™ sample tubes. A straightforward conversion into μg haemoglobin per gram of faeces enables non-system-dependent comparison.
μg Hb / g stool = ((ng Hb/ml) x (buffer volume in the system, in ml))/(mass of faeces in mg)
Conversion table for OC-Sensor™ concentrations:
ng Hb /mL Buffer (OC-Sensor™) µg Hb/g Stool 10 2 50 10 100 20 250 50 1000 200
The OC-Sensor™ instruments utilize a unique squeeze function that pass liquid though the integrated filter into the collection cup portion of the sample bottle. Once released, the sample aliquot is pulled back into the buffer chamber as the bottle regains its shape. This makes the sample bottles spill proof when removed from the machine (without having to recap) and placed into the refrigerator for storage and prevents the sample drying out or becoming concentrated.
In the event that repeat measuring of the sample is required, the analysis device includes a “re-test” function. In this mode the test can be repeated under the same conditions, without making a new hole in the aluminium foil.
Normally two barcode labels are given on the OC-Sensor™ sample bottles: the top one can be stuck onto the voucher/test request form, while the bottom barcode is left for the analysis and safe labelling of the sample on the sample bottle.
In the UK, most OC-Sensor™ sample bottles are supplied without barcodes so that the laboratory can use its own internal barcoding for data administration, applied directly to the bottle.
Very specific polyclonal, anti-human HbA0 rabbit IgG antibodies are used, bonded to latex particles.
When the polyclonal antibodies detect the epitopes of antigens and bind to them, this causes an agglutination reaction.
Quantitative FIT tests offer an automated, quality-assured and precise evaluation under standardised conditions in a medical laboratory. Interpretation is objective and thereby rules out misinterpretations. In the case of quantitative FIT the cut-off score (borderline score) can be adjusted and the results are delivered in a regards to quantity of haemoglobin in the stool sample, usually in µg of haemoglobin per g of faeces.
Qualitative FIT tests deliver a qualitative “Yes/No” result relating to a borderline score, which is set by the respective test manufacturer and is difficult to verify. This fixed cut off makes them impossible to adapt to local requirements and therefore not suitable for national screening program use. Analysis of rapid qualitative tests can be performed in the laboratory of the GP practice. However, the visual interpretation of results by a colour change reaction is subjective and prone to false positive reporting. There are a multitude of tests on market that vary greatly in their sensitivity, specificity and limit of detection.