Clinical Chemistry / DIGOXIN-LIKE IMMUNOREACTIVE SUBSTANCES

Endogenous digoxin-like immunoreactive substance (DLIS) was first reported in volumeexpanded dogs. Its presence has been confirmed in blood, urine, and other body fluids. Elevated DLIS concentrations are encountered in patients with volume-expanded conditions such as uremia, essential hypertension, liver disease, and preeclampsia. DLISs cross-react with antidigoxin antibodies and falsely elevate serum digoxin concentrations, interfering in interpretation of results for therapeutic digoxin monitoring. Falsely lower digoxin values due to the presence of DLISs have been reported. The association of DLISs with volume expansion led to speculation that they could be natriuretic hormones. Several structures have been proposed for DLISs, including nonesterified fatty acid, phospholipid, lysophospholipid, bile acid, bile salt, and steroid. Exogenous DLISs can be found in serum after ingestion of various Chinese medicines and therapy with spironolactone, canrenone, or potassium canrenoate. Like endogenous DLISs, exogenous DLISs interfere with serum digoxin assays, complicating therapeutic digoxin monitoring. However, most reported endogenous and exogenous DLISs are strongly protein-bound while digoxin is weakly protein-bound. Therefore, interference of both endogenous and exogenous DLISs in serum digoxin measurement can be eliminated by monitoring digoxin concentrations in the protein-free ultrafiltrates. Discovery of Endogenous Digoxin-like Immunoreactive Substances Digoxin is a cardiac glycoside used most frequently to increase the adequacy of circulation in patients with congestive heart failure and to slow the ventricular rate in the presence of atrial fibrillation and flutter by blocking the atrioventricular node. After the discovery of endorphins, the endogenous equivalent of opiates, there was a hypothesis for the presence of the endogenous equivalent of cardiac glycosides. It was further hypothesized that antidigoxin antibody may be able to detect the presence of digoxin-like immunoreactive substances (DLISs) in body fluids. Gruber et al1 first demonstrated the presence of endogenous DLISs in 1980, in volume-expanded dogs. Then Craver and Valdes2 reported an unexpected increase in the serum digoxin concentration in a patient with renal failure and already receiving digoxin. The apparent serum digoxin level was still present after discontinuation of digoxin therapy.2 Balzan et al3 also confirmed the presence of DLISs in human plasma and urine. DLISs were found in various human body fluids and tissues including cord blood, placenta, amniotic fluid, bile meconium, cerebrospinal fluid (CSF), and saliva.4,5 DLISs cross-react with antidigoxin antibodies and inhibit Na+, K+–adenosine triphosphatase (ATPase). Detection of DLISs in Body Fluids DLISs can be detected in serum and other body fluids by using commercial immunoassays for digoxin, taking advantage of the cross-reactivity of DLISs with antidigoxin antibody. Clinical Chemistry / REVIEW ARTICLE Am J Clin Pathol 2002;118:132-140 133 © American Society for Clinical Pathology Apparent digoxin concentrations as detected by radioimmunoassay (RIA) digoxin assays had been reported by several investigators in patients not receiving digoxin.6,7 Some of those RIA digoxin assays later were discontinued owing to high interference from DLISs. Early reports also indicated cross-reactivity of DLISs with the fluorescence polarization immunoassay (FPIA) marketed by Abbott Laboratories, Abbott Park, IL.8 Although many investigators used commercially available digoxin assays for detecting DLISs in body fluid, other approaches also have been documented. Panesar9 used bufalin as an antigen and developed polyclonal antisera for detecting DLISs. Lin et al10 developed a polyclonal antibody–based ouabain enzyme immunoassay for detecting DLISs, and they also developed a Fab of the antidigoxin antibody–based enzyme immunoassay for this purpose. They concluded that a polyclonal antibody–based ouabain assay was more efficient for detecting DLISs in human blood.10 More specific highperformance liquid chromatographic techniques using reverse phase columns also had been used for detecting DLISs in biologic fluid.11,12 However, these techniques are time consuming and technically more difficult than automated immunoassays.