How are Ratio calculated?
The instrument measures a fluorescence signal which is given in relative units (RU). RU-values are calculated into the number of IgG antibodies (µg/l) with help of the calibration curve. The calibrators have a defined concentration of antibodies in µg/l. These calibrator concentrations are traceable to WHO standards.
EliA Symphony and EliA CTD Screen are ENA Screening tests with all antigens in one well. This does not allow an exact quantification. However, to obtain a rough idea how positive a patient sample is IDM calculates the concentration in Ratio. The value tells you how many times above the cut-off the concentration is - a result of 2.0 Ratio is 2x the cut-off.
Equivocal zone is 0.7 to 1.0 Ratio (for EliA CTD Screen and EliA Symphony).
A rough rating to assess the positivities: 1.0 to 4.0 is low positive, 4.0 to 10.0 is moderate and more than 10 is strong positive. If only one antibody is positive, it won't go so high as if several positivities add up.
Apart from that difference, the calculation of Ratio in IDM is done in the same way as the calculation of EliA Units/ml (see here »). It is not calculated in the same way as Ratio for Varelisa Screens, where the ratio of patient OD towards Cut off OD is calculated (see below).
ug/l x Dilution factor x Correction factor x Lot Specific factor = Ratio
Dilution factor: This result has to be set into relation to the dilution of the sample. EliA Symphony has a default instrument dilution of 1:100. Example: The instrument found 300 RU and "translates" this with help fo the curve into 9 µg/l. Accordingly, the "real" result for the sample is 900 µg/l.
standard dilution EliA Symphony: 1:100
standard dilution EliA CTD Screen: 1:10
for EliA Symphony: 0,001 (Phadia 100) 0,00112 (Phadia 250).
for EliA CTD Screen: 0,01 (Phadia 100) 0,01 (Phadia 250).
Lot-specific factor / Code:
This factor allows for the correction of slight signal differences between lots, and it is generally used to convert concentrations in µg/l into U/ml or Ratio. The factor is included as a two-digit code in the barcode.
Example of EliA Calculation