The Performance of the Point of Care Test (POCT) i-CHROMA Ferritin Method and other Methods Enrolled in the RIQAS

Ferritin is present in all cells of an organism, present in plasma and serum and considered a reflection of iron stores. Ferritin can be measured using enzyme linked immune sorbent assay (ELISA), radioimmunoassays (RIA), immunoradiometric assays (IRMA), agglutination methods and chemiluminescent methods. The i-CHROMATM Ferritin is a point of care test (POCT) lateral flow chromatography, fluorescence immunoassay (FIA) for the quantitative determination of ferritin in serum or plasma. In this study, the performance of the i-CHROMATM Ferritin method was evaluated with other laboratory ferritin methods using Cycle 41 from the Randox International Quality Assessment Service (RIQAS). The i-CHROMATM Ferritin correlated well with other laboratory ferritin methods: Abbott Axsym, Abbott Architect Chemiluminescence, Beckman AU400/600/640/2700/5400, bioMerieux, VIDAS, Roche Cobas C501/502/701, Roche Cobas E601/602, Siemens Centaur CP, Siemens Centaur XP/XPT/Classic, Siemens Dimension, Siemens/DPC Immulite 2000/2500, Siemens/DPC Immulite 1000, Beckman DxI 600/800, Roche Integra, DiaSorin, Liaison, Monobind Inc. ELISA, Roche Cobas 4000/c311, Roche Modular E170/Cobas e601/e602, Beckman, Access/LXi725 and Ortho Vitros 3600/5600/ECi. The results of this comparative study have shown that the i-CHROMATM Ferritin estimations were consistently lower ranging between +6% to -125%, but there was very good linear regression and correlation. The laboratory methods that were closest to the estimations of the i-CHROMATM Ferritin method were the Monobind Inc ELISA(Y=1.1082X 31.819; r2=0.9571), Ortho Vitro 3600/5600/ECi(Y=0.9805X-21.052; r2=0.9346) and the Beckman Dxl 600/800(Y=0.9012X-16.848; r2=0.9171). In summary, the i-CHROMATM Ferritin method correlated well with other laboratory methods, however, there was a significant negative bias seen. Therefore, it is important to take these negative biases into consideration when applying reference values.


Introduction
Ferritin is present in all cells of an organism, present in plasma and serum and considered a reflection of iron stores [1,2]. Ferritin can be measured in serum, plasma and erythrocytes using ELISA, RIA, IRMA, agglutination methods and chemiluminescent methods [3]. The i-CHROMA™ Ferritin is a point of care (POC) lateral flow chromatography, FIA for the quantitative determination of ferritin in serum or plasma. The analysis performance of the i-CHROMA™ Ferritin method determined by the manufacturer showed an analytical sensitivity of 4.51 ng/mL, an intra-assay precision (6.54% at 15 ng/mL, 2.73% at 150 ng/mL and 17.6% at 450 ng/mL), an inter-assay precision (6.22% at 15 ng/mL, 1.20% at 150 ng/mL and 1.58% at 450 ng/mL) [4]. We have extensively evaluated the comparative performance of the Boditech immunoassay POCT device i-CHROMA™ with other laboratory methods using samples protein in the sample. The sample is then applied onto a test strip and the fluorescence labelled antigen-antibody complex is captured by a second antibody embedded in the solid phase. The signal intensity of fluorescence of the captured complex is directly proportional to the amount of ferritin present and thus allows for the calculation of sample ferritin concentration and the result is displayed on the reader as nanograms per millilitre (ng/mL). A fluorescence-labelled control protein is included in the reaction and the intensity of the control line is measured as a quality check.

Ferritin method
The assay was performed following the manufacturer's instructions 1. Transfer 30 μL of sample using a transfer pipette to a tube containing the detection buffer.
2. Close the lid of the detection buffer tube and mix the sample thoroughly by shaking it about 10 times.
3. Pipette out 75 μL of a sample mixture and load into a sample well in the cartridge.
4. Leave the sample loaded cartridge at room temperature for 10 minutes.
5. Insert the sample loaded cartridge into the cartridge holder of the i-CHROMA™ Reader.
7. Read the result on the display screen.

Discussion and Conclusion
The results of this comparative study have shown that the i-CHROMA™ Ferritin estimations were consistently lower than the estimations of all the laboratory ferritin methods registered in the RIQAS (Abbott Architect Chemiluminescence, Beckman AU400/600/640/2700/5400, bioMerieux VIDAS, Roche Cobas C501/502/701, Siemens Centaur CP, Siemens Centaur XP/XPT/ Classic, Siemens Dimension, Siemens/DPC Immulite 2000/2500, Siemens/DPC Immulite 1000, Beckman DxI 600/800, Roche Elecys/ Cobas e411, DiaSorin, Liaison, Monobind Inc. ELISA, Roche Cobas 4000/c311, Roche Modular E170/Cobas e601/e602, Beckman, Access/ LXi725, and Ortho Vitros 3600/5600/ECi). The negative bias observed with the i-CHROMA™ method was quite high. The estimation of ferritin is dependent on which ferritin isoforms are measured as there are at least 2 isoforms present, one distributed predominantly in the liver and the other predominantly in the spleen [12,13]. This might be one of the reasons why the i-CHROMA™ method estimates are significantly lower than the other methods and warrants further investigation. However, there were some laboratory methods that were reasonably closer to the estimations of the i-CHROMA™ method such as the Monobind Inc ELISA, Ortho Vitro 3600/5600/ECi and the Beckman Dxl 600/800. In addition, the i-CHROMA™ method also had very good linear regression and correlations with all the methods and appeared to have the best correlation and linear regression with the following methods: Monobind Inc ELISA (Y=1.1082X-31.819; r 2 =0.9571), Ortho Vitros 3600/5600/ECi (Y=0.9805X -21.052; r 2 =0.9346) and Beckman DxI 600/800 (Y=0.9012X-16.848; r 2 =0.9171), there may be some similarities between these assays and i-CHROMA™ method. In the product leaflet, the manufacturers have comparative data between the i-CHROMA™ Ferritin and the mini VIDAS method, they had a linear regression and correlation Y=0.99198X+0.56317; r 2 =0.9897. In this study, the i-CHROMA™ Ferritin method had a linear regression and correlation with the bioMerieux VIDAS method of Y=0.73X-18.647: r 2 =0.8983 [5], so the negative bias might not be consistent. A recent systematic review and meta-analysis concluded that methods that determine ferritin are comparable but there is no reference method and the use of International reference material such as the WHO standard should be used to calibrate all commercial assays [3]. In summary, the i-CHROMA™ Ferritin method correlated well with the laboratory methods, however, there was a consistent underestimation seen compared to most laboratory methods apart from the following methods: Monobind Inc ELISA, Ortho Vitro 3600/5600/ECi and the Beckman Dxl 600/800.Therefore,

Method
Linear Regression  it is important to take these negative biases into consideration when applying reference values, or should, preferably, be solved with the use of international reference material to help calibrate the method.