Bioanalytical science is never absolute is almost always relative.
- Its two different methods. The LC MS method is calibrated to the CDC standard. The IA may or may be not calibrated to the CDC standard, although if it was run at the same lab i would assume it is.
- Both have a variability of about 10 to 15%.
So when the ‘true’ T value is at 1000 ng/dl the measured can be somewhere between 930 to 1070.
So the difference between the methods is to be expected and nothing to worry about.