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For some ELISA based antibody tests (e.g. h-tTg antibody test), labs report units as RU/mL or U/mL. Also different labs have different cut off (normal range) values.

I understand that different kit manufacturers recommend different ranges because the units are not standardized (e.g. IU/mL) for enzyme activity concerned. But I didn't understand the difference between RU, U and AU

Question Is there a difference between RU/mL, AU/mL and U/mL? Are the figures comparable across reports from different labs (which may be using different kits)?

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  • $\begingroup$ No actually. U is definitely not IU. I am referring to ELISA tests where there is no international standards yet. For example, IgA tTg antibody test. I know AU is Arbitrary Units. RU is either Relative or Reference Unit. U is simply unit. Doubts is what's the difference between these. $\endgroup$ – Dayne Jul 16 at 4:11
  • $\begingroup$ Thanks for informative comment. I am not from biology background so your comment helped. Now what I meant by standardization is that what conjugate to use and incubation times are not standardized for some tests. Because of this, extinction coefficients are not known. So the lab depends on calibrators given in the kit. Calibrators have specified units and using that curve is fitted. Based on fitted curve sample's concentration is derived. Now, from what i read, manufacturers give their own units. But some give RU some U some AU. For tests where standard reaction conditions are defined IU. $\endgroup$ – Dayne Jul 16 at 11:53

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