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42. Pre-analytical considerations of the hormone tests.
- Measured by serum in plain tube (red) or gel tube (yellow)
- Small peptide hormones
- Unstable, transported quickly and on ice
- TRH
- Renin
- Amino acid derivatives
- T3, T4
- Only free fraction is measured
- Protein binding influences measurement
- Adrenalin
- Steroids
- Cortisol
- Aldosterone
- Sex hormones
- Both fractions are measured
- Binding protein should also be measured
- Biological variation
- GH – pulsatile release
- Cortisol – influences by stress, diurnal rhythm
- Should be measured in the morning
- Log-linear relationship – small change in TRH can cause significant change in TSH
- Analytical variations
- Measured by immunoassays
- Immunoassay interference
- Heterophilic antibodies – low affinity antibodies
- Anti-animal antibodies
- Autoantibodies
- Antibodies from treatment
- Rheumatoid factor
- Matrix effect
- High level of molecules in plasma -> molecules can cover epitopes of analyte -> underestimate level of analyte
- High-dose hook effect
- More analyte than capture antibody -> analyte binds to detection antibody without being captured -> detection antibodies depleted -> fewer captured analytes can be labelled -> false negative result