Stability of indocyanine green in human serum stored at -20 and -70 degrees C.

The stability of indocyanine green (ICG) in human serum after storage at -20 and -70 degrees C for an extended time was studied. Serum samples were fortified with ICG at 6.5, 2.0 and 0.3 microgram ml-1, aliquoted, and frozen at -20 or -70 degrees C. The analytic methodology used an internal standard and separation on a reverse phase liquid chromatographic column with UV detection at 240 nm. Samples were prepared for analysis by protein precipitation with acetonitrile. The assay was stability indicating as shown by the complete loss of the ICG peak upon storage at -20 degrees C for 8 weeks. ICG was not stable, as determine by a t test, for 1 week in human serum when stored at -20 degrees C. Storage at -70 degrees C improved stability but the ICG concentration began to fall steadily after 4 weeks of storage.