FIT (Faecal immunochemical test) is an alternative name for Immunochemical faecal occult blood test (iFOBT), a class of faecal occult blood tests. Faecal Occult Blood Tests (FOBTs) are tests for blood or blood products. They use blood as an indicator of the presence of tumour {1}.

A wide range of qualitative and quantitative iFOBT tests is presently available, with varying levels of sensitivity and specificity. They all use antibodies raised against human haemoglobin (Hb) to detect human blood present in faeces.

Similar to g FOBT, participants collect one or more stool samples. The sampling procedure varies: different sampling systems are available (wooden sticks, brushes) and samples may be applied either to a card (dry method) or to place into a vial (wet method). Samples can be analysed either using automated systems in the laboratory (for some manufacturers) or are read by the naked eye with a positive result indicated by a colour change on a strip {2}. The automated systems can be quantitative or qualitative. Qualitative tests produce a dichotomous result, with individuals categorized as either positive or negative if the amount of Hb in the faecal sample is above or below a specific analytical detection limit set by the manufacturers. In quantitative tests, specificity can be determined by the user {3}. The most frequently used values are 75 or 100 ng/mL. This is important due the fact that by increasing the positive cut-off limit, the test sensitivity and positivity rate decreases and specificity and positive predictive values for CRC detection increase. FIT kits with a visual result are designed as point-of-care devices and are qualitative. They can be adopted for use in clinical laboratories for population-based screening; however they retain a more manual approach than the automated systems.

In their report on the comparison of FITs vs. g FOBTs for population-based colorectal cancer screening in Ontario, CA {4} the FIT Guidelines Expert Panel has considered test processing in laboratories more suitable that point-of-care systems for a population-based screening program. Similarly, the NHS Centre for Evidence-based Purchasing in the UK adopted certain criteria that the iFOBTs had to meet in order to be evaluated for the CRC screening programme in the UK {5}. Only automated systems were included in the evaluation. Regarding staff requirements, more technical knowledge will be required than for gFOB tests. If the increased packing material is required to meet postal regulation, unpacking of iFOBT liquid sampling devices may take longer than for gFOBT.

Potential advantages and disadvantages of iFOBT are given in Table 1.

Table 1. Potential advantages and disadvantages of iFOBT (5)

Three iFOBT are presented here, as three analytical platforms using the three sample collection devices: OC-Sensor/OC-Sensor Diana & OC-Sensor Micro, Hem-SP/MagStream HT, FOB Gold/SENTiFOB analyser.  In brief {6}:

•  OC-Sensor measures human Hb concentration in faeces samples by latex agglutination using polystyrene latex particles coated with polyclonal anti haemoglobin Ao antibodies. One sample only is recommended as number of separate samples used for assessment. The Diana has a memory capacity for 100 000 test results, with speed of 280 samples per hour analysis. Ten mg of faeces is recommended as quantity that should be collected by sampling device. Advantage is that this test is CE marked for a user defined cut-off (default setting 100 ng/mL), with 20 ng/mL in buffer as limit of detection. The faecal sample is collected into the OC-Auto sampling bottle 3. Analysis should be performed as soon as possible after sample collection; in case of any delay the sample collection bottles should be stored at 2-10C;
• Hem-SP/MagStream HT (MagStream Hem-Sp®) is based on magnetic particle agglutination (the magnetic particles are coated with rabbit anti-human Hb antibodies). Two samples are recommended as number of separate samples used for assessment. One disadvantage is that this method is qualitative, and has a non-adjustable cut-off at a Hb concentration equal to or greater than 20 ng/ml. In the presence of human haemoglobin, the particles remain aggregated in a spot with minimal change (positive result). In the absence of human haemoglobin, particles flow down the slope (negative result).  MagStream HT, an automated instrument which holds 400 samples has a memory capacity of 2 million test results, with speed of analysis of 960 tests per hour (MagStream HT). 0.3 mg of faeces is recommended as quantity should be collected by sampling device. The faecal sample is collected into the NEW HEMTUBE. Analysis should be performed as soon as possible after sample collection; in case of any delay the sample collection bottles should be stored at 2-10C. Freezing must be avoided;

The FOB Gold use an antigen-antibody agglutination reaction between human haemoglobin and polyclonal anti-human haemoglobin antibodies coated on polystyrene particles. The total reading time is 8 minutes, with speed of analyses of 75 tests/hr

• (SentiFOB). The FOB Gold reagents can be used on any suitable immunoassay automated analyser although the manufacturer provides the SENTiFOB analyser. Advantages of this test is that this test is CE marked for a user defined cut-off, with limit of detection of 14 ng/mL buffer, and measuring range of 15-1000 ng/ml. The faecal sample is collected into the FOB Gold tube. Analysis should be performed as soon as possible after sample collection; in case of any delay the sample tubes should be stored at 2-8 C.

Some of products characteristics are presented in Table 2.

 Table 2. Product characteristics of OC-Sensor/OC-Sensor Diana & OC-Sensor Micro, Hem-SP/MagStream HT, FOB Gold/SENTiFOB analyser {5-9}.

Different authors {5-8} have compared the analytical performance of 3 iFOB tests: OC-Sensor/OC-Sensor Diana & OC-Sensor Micro, Hem-SP/MagStream HT, FOB Gold/SENTiFOB analyser. With regard to reproducibility and temperature stability, OC-Sensor performed better than Magstream and far better than FOB Gold. For all tests, variability was essentially related to sampling. Detected Hb levels were substantially lower for all tests at temperatures above 20 C. This loss is more important for FOB Gold than for other two tests. Some suggestion are made, like delay between sampling and test processing should be reduced to 3 days, or CRC screening programs should be stopped during the summer in countries with long period of very high temperatures >30 C. Patients should be advised to store faecal samples in the refrigerator at home before forwarding them by post.

Rossum et al. {10} reported that delay in sample return increased false negative iFOBT because of Hb degradation. Compared with no-delay, the adenoma detection rate was significantly decreased after >5 days delay (OR 0.6; 95% CI 0.4-0.9).

Sentinel Diagnostic Spa, manufacturer of FOB Gold NG test was only one who responded on our Survey for Manufacturer and sent documentation for their FIT products. The product leaflet of SENTiFIT pierce Tube (collection tube) writes that the human haemoglobin extracted from the feces sample and obtained according to the recommended collection procedure is stable for 14 days at 2-8 C or 7 days at 15-30 C protected from direct light.

The authors of the NHS Centre Evaluation report in 2009 {5}, concluded that the OC-Sensor/DIANA analyser was the most suitable system for the English CRC population-based screening programme. They reported that even when the faecal collection devices were used by experienced evaluation staff, none of the sampling devices gave reproducible results, contributing to the overall imprecision of the methods. Manufacturers claims for stability of the Hb in faecal samples added to the collection devices and stored at room temperature (23C-26C) prior the analysis were confirmed for the OC-Sensor/DIANA method only. Analytical methods comparisons are presented in Table 3.

Table 3. Analytical methods comparisons for 3 iFOB tests: OC-Sensor/OC-Sensor Diana & OC-Sensor Micro, Hem-SP/MagStream HT, FOB Gold/SENTiFOB analyser {5}

According the authors of this report {5}, all 3 analytical platforms are easy to operate and maintain once appropriate training has been received. For all three tests, the liquid samples stored at room temperature for more than three days are not suitable for analysis, due to deterioration of any Hb present. This should be taken in account when sending samples via different MSs postal systems (should be returned within this time). The OC-Sensor/DIANA analyser was the most suitable system for the UK bowl cancer screening programme, despite limited reagent, wash and waste capacity (regular attention will be required in a busy screening laboratory). The FOB Gold/SENTiFOB analyser has a very low sample throughput. The HemSp/MagStream HT has a non-adjustable cut-off, the method is not CE marked for quantitative measurement of human Hb. The system gave negative results for samples that were positive by other methods.

For all three tests sample collection devices should be stored between 2 and 10 C in case of any delay in analysis, so refrigerated storage space is required. The number of analysers required will depend on the laboratory workload; number required for a 5,000 sample per day workload will be 1 for HemSp/MagStream HT; 5 for OC-Sensor/DIANA, and 15 SentiFOB and 1 chemistry analyser for FOB Gold/SENTiFOB. Specialist training is provided by the manufacturers (instructions on routine use and maintance of the analyser). Staff will be required to authorise each batch of results generated from the iFOBT analysers and transfer the data to the bowel cancer screening programme database, if such exists {5}.