Digoxin-like immunoreactive substance: monoclonal and polyclonal RIA and FPIA compared.

Digoxin-like immunoreactive substance (DLIS) was measured in 34 samples obtained from subjects not receiving digoxin: 10 uremic, 10 third trimester pregnancy, seven cord blood and seven normal. DLIS concentration was measured by four commercial polyclonal radioimmunoassay (RIA) systems: Clinical Assay (CA), Corning Immophase (CI), Diagnostic Products Double Antibody (DP), Kallestad Quantitope (KK), a monoclonal antibody (MA) assay and a Fluorescence Polarization Immunoassay (FPIA). In general, the cord blood samples were richer in DLIS. Digoxin immunoassays, MA and DP showed minimal interference by DLIS in all samples, whereas FPIA and CA exhibited the maximal cross-reactivity with DLIS. In cord blood samples, mean +/- SD DLIS concentration ranged from 0.41 +/- 0.13 (by CA) to 0.034 +/- 0.02 ng ml-1 as measured by MA assay. In uremics, the mean DLIS concentration was below the detection limit of all RIA assays. The FPIA method showed a higher degree of cross-reactivity to DLIS, especially in the cord and pregnancy samples (0.42 +/- 0.13 and 0.4 +/- 0.14 ng ml-1, respectively). DLIS in uremics was below the FPIA detection limit of 0.2 ng ml-1. Overall, the degree of interference by DLIS in decreasing order was FPIA greater than CA greater than CI greater than or equal to KQ greater than DP greater than or equal to MA. The cord blood samples were re-analysed by FPIA (Digoxin-II assay) 4 months later, resulting in 2-4-fold higher DLIS concentrations for these samples. This appears to be due to the substitution of 5-sulphosalicylic acid as a protein precipitating reagent and this effect may have been accentuated by freeze-thaw cycles.

Production and characterization of specific antibody for radioimmunoassay of procainamide.

The production and characterization of a specific antibody for use in the radioimmunoassay of procainamide are described. Cross-reactivity was measured by a nonequilibrium competitive procedure. Procainamide analog concentrations resulting in 50% inhibition were: procainamide, 1.59 nmoles/ml; N-acetylprocainamide, 3.55 nmoles/ml; a propyl analog of procainamide, 398 nmoles/ml; procaine, 316 nmoles/ml; lidocaine, greater than 8000 nmoles/ml; and practolol, greater than 16,000 nmoles/ml. Variations in the ability to inhibit binding of labeled procainamide were related to structural similarities and differences. The affinity constant of the antibody for procainamide was 2.9 x 10(8) liters/mole as measured from a Scatchard plot. The assay allows the direct measurement of procainamide in a 0.1-ml aliquot of diluted serum. The advantages of this method over currently available techniques are its sensitivity, specificity, and simplicity. Furthermore, prior extraction of serum samples is not required. As little as 1 ng of drug/ml of serum can be detected by this method. The accuracy and precision were determined by adding known amounts of procainamide to human serum and then assaying five replicates of each concentration. The within-day and between-day coefficients of variation were 2 and 5%, respectively. The proposed method was used to determine the serum concentration after an intravenous dose of procainamide. A comparison of the radioimmunoassay results with values obtained by a GLC procedure showed excellent agreement.