RESOLVING DISCREPANT DIGOXIN RESULTS - A CASE-STUDY INVOLVING MULTIPLE INTERFERENCES

Clinicians and laboratorians are often faced with clinically discrepan t digoxin results that are difficult to interpret. We present a clinic al case example in which a serum sample gave widely discrepant digoxin values by six immunoassays, ranging from 3.4 to 25.6 ng/mL. We employ ed ultrafiltration, diafiltration, incubation kinetic analysis, and ch romatographic techniques to evaluate and characterize the nature of th e discrepancy, and we discovered multiple causes, A strongly protein-b ound substance present in serum increased the fluorescence background, Previous but unknown administration of Digibind to the patient contri buted to the discrepancies, and a trace of digoxin-like immunoreactive factor was present, High-performance liquid chromatographic analysis revealed the sample also contained a total digoxin concentration of ap proximately 6.0 ng/mL, We concluded that in addition to true digoxin, the discrepant sample had multiple interferents present causing signif icant inconsistencies in the measured digoxin values, This study prese nts a step-by-step rigorous approach with general application to resol ving discrepancies observed when measuring digoxin in serum, The appro ach is adaptable to discrepancies observed in other TDM immunoassays a nd serves as a clinical pathology teaching tool for resolving discrepa nt lab results.