Background: Significant positive bias of bilirubin in the Trinder
salicylate method on automated analysers has been reported. Because the fluorescence
polarization immunoassay (FPIA) for salicylate is also widely used in the clinical
laboratory, we studied the potential interference of bilirubin in the salicylate
FPIA.
Methods: Salicylate serum pools (three different pools) were prepared
from patients receiving salicylate. We also prepared a normal serum pool containing
no salicylate and serum pools containing no salicylate but elevated bilirubin.
Aliquots of one salicylate pool were supplemented with various concentrations of
bilirubin (42.8- 427.5 µmol/L) and salicylate concentrations were measured by the
salicylate FPIA (TDxFLx and AxSYM analysers). We also assayed these specimens with
the Trinder salicylate method, using both Synchron LX and Hitachi 917 analysers for
comparison with the results obtained by the FPIA method. In another experiment,
aliquots of the two other salicylate pools were supplemented with various
concentrations of bilirubin (42.8-684.0 µmol/L) in order to further study the effect
of very high bilirubin concentrations on the salicylate FPIA. We also added known
amounts of salicylate to serum pools containing elevated bilirubin but no salicylate
and measured salicylate using the FPIA in order to study the recovery of salicylate
in the presence of elevated bilirubin concentrations.
Results: The FPIA showed minimal interference from bilirubin. We also
observed good recovery of salicylate when specimens high in bilirubin but containing
no salicylate were supplemented with known amounts of salicylate and the FPIA was
used for the measurement of salicylate concentration. However, we observed falsely
low salicylate concentrations with the Trinder method using the Synchron LX (primary
wavelength 560 nm, secondary wavelength 700 nm) analyser and falsely increased
salicylate concentrations using the same reagent but the Hitachi 917 (primary
wavelength 546 nm, no secondary wavelength) analyser in the presence of elevated
bilirubin levels compared with the FPIA results.
Conclusion: We conclude that the FPIA for salicylate is not affected by
high bilirubin concentrations up to 427.5 µmol/L.
