False-positive result in Limulus test caused by Limulus amebocyte lysate-reactive material in immunoglobulin products

Limulus amebocyte lysate (LAL)-reactive material other than endotoxin was detected in the plasma and urine of patients after intravenous immunoglobulin therapy. Thirty-seven vials of six different immunoglobulin products were analyzed for the LAL-reactive material by combined use of a conventional chromogenic Limulus test and a chromogenic endotoxin-specific test. The amount of LAL-reactive material in reconstituted immunoglobulin solutions ranged from a mean (standard deviation) of 10.2 (2.1) to 2,448.1 (988.9) pg/ml, and there were statistically significant differences among the six brands. The levels of LAL-reactive material in plasma increased in proportion to the amounts contained in the immunoglobulin products administered. The material accumulated in the blood with repeated administration. Urinary excretion of the material was less than 5% of the total amount administered. Such material seems to be derived from the cellulose-based membranes used during preparation of the blood products. Thus, interpretation of Limulus test results of patients receiving immunoglobulin therapy requires special consideration.