Thyroid-function testing with the TDx: interference from endogenous fluorophore.

One agent used by ophthalmologists to aid in funduscopic examinations is sodium fluorescein (“Funduscein,” Cooper Vision). We recently received a patient’s specimen for measurement of thyroxin (T4) and T3-uptake, drawn one day after she had been examined by an ophthalmologist. Owing to the high background fluorescence (62 580 arb.units) and low background polarization (24 mPol), no results were printed for either of these tests on the Abbott TDx analyzer. However, there was significant carryover by the TDx, so that the next specimen gave falsely high values (179 jg/L vs 62 g/L for T4; 44% vs 39% for T3-uptake). Even though modestly-increased blank intensities (1450 and 389 intensity units for thyroxin and T3-uptake, respectively) were noted, no error codes were printed by the instrument. A carryover check was performed and results were within normal limits. Values obtained for thyroid-function tests on the specimen with endogenous fluorescein (patient 1) and for the succeeding specimen in the TDx sample carousel (patient 2) are summarized below.