A method is disclosed whereby the concentration of a blood substitute, such as cross-linked hemoglobin, in a serum or plasma specimen is rapidly and accurately identified and quantified. The method further takes the measured concentration of the blood substitute and uses it to correct for its effect, if any, on a measured analyte concentration, e.g., serum/plasma total protein. Further, the method allows for the determination of the concentration of true hemoglobin in the presence of blood substitutes. The method is carried out in respect of samples contained in a primary or secondary labelled tube, or a pipette tip used to dispense serum or plasma in a blood analyzer.