Influence of monoclonal immunoglobulins in three different methods for inorganic phosphorus

Three methods for the determination of inorganic phosphorus were compared in order to evaluate the frequency of falsely high concentrations of inorganic phosphorus in 102 sera containing monoclonal immunoglobulins. The results for the three methods using neat sera were compared with those obtained from protein-free filtrates of the same sera. For the neat sera erroneously high results were encountered with the molybdate method in about 10% (11/102) of the cases. In contrast neither the enzymatic method nor the modified molybdate method gave erroneous results. The presence and magnitude of error of the molybdate method bore no relationship to the type or the concentration of the monoclonal immunoglobulins. The precipitation of monoclonal immunoglobulins by the strongly acidic molybdate reagent, leading to erroneous results, can not always be prevented by the addition of a detergent. Nor can the problem be solved by a simple two- or threefold dilution.