Measuring carbonic anhydrase IX as a hypoxia biomarker: differences in concentrations in serum and plasma using a commercial enzyme-linked immunosorbent assay due to influences of metal ions

There is increasing interest in measuring the soluble forms of carbonic anhydrase IX (CA IX) in blood as a marker of hypoxia for prognostic purposes or for predictive use in therapeutic trials in various cancers. Following our initial observations of marked differences in the measured concentrations of CA IX in EDTA plasma versus serum, we sought to investigate these further in order to determine their effects on results in published studies and to ensure accurate measurement in future studies.