Samples from two pools were sent 10 times to 27 laboratories for assay ofdigoxin.One pool contained digoxin 2.60 nmol/l in normal plasma (SP); the other was pooled plasma from patients treated with digoxin (PP). Ten radioimmunoassay (RIA) methods were used. The mean of SP assays was 2.59 nmol/l, not significantly different from 2.60 nmol/l. The mean of PP determinations was 2.46 nmol/l. Within each of the 10 assay rounds, the concentrations showed an almost twofold variation and S.D. averaged 0.33 nmol/l and 0.31 nmol/l for SP and PP respectively.Significant differences (P
The use of multicomponent solvents in physico-chemical measurements on polymers renders necessary some caution in the interpretation of the experimental data. Especially, in molecular weight determination by light scattering, it is necessary t o use a proper value for the refractive index increment. Such a value may be obtained by applying a correction to the conventional refractive index increment, or more directly from differential refractive index measurements between a solution and solvent brought to dialysis equilibrium. Although the theoretical background of this procedure has been known for some time1-I2) there still seems to be some confusion about its application in actual experiments. In the present work refractive index increments were measured for some polysaccharides in multicomponent aqueous solutions. The measurements were carried out both with dialyzed and undialyzed solutions.
ExperimentalThe refractive index measurements were carried out with a differential prism refractometer, which was built a t this institute and has been described elsewhere13). The prism cell was calibrated with aqueous solutions of Na,SO,, using the refractive index values of K R U I S~~) .The dialysis experiments were carried out in a cell described in15), using commercial cellophane membranes. All refractive index measurements and dialysis experiments were carried out a t 25°C.
MaterialsThe following substances were used in the measurements.Cellulose. The sample was obtained from high D P (m5500) cotton linters, which were subjected to a mild chlorite bleach and subsequently degraded by acid hydrolysis to about DP = 1100.Dextran. This was a commercial fractionated sample (from Pharmacia, Uppsala, Sweden) with the following characteristics: [Yj]HzO = 0.43 dl/g, DP, = 1460.
We evaluated four commercial radioimmunoassay kits for digoxin. We assayed a standard plasma containing digoxin, 2.0 microgram/L, and samples from patients receiving digoxin, with use of the kits and of a bioassay, the 86Rb-uptake inhibition technique. Intra-assay precisions differed significantly. Computer-calculated 95% confidence intervals for the radioimmunoassays averaged 0.4 to 0.6 microgram/L at the proposed toxic threshold of 2.0 microgram/L; the corresponding value of the 86Rb assay was 0.75 microgram/L. Digoxin in the standard plasma was overestimated with three of the kits (means: 2.40, 2.56, and 2.59 microgram/L) but was assayed accurately by the 86Rb technique and by one kit. This same kit gave a significantly lower mean (1.07 microgram/L) for the patients' samples then did the other three kits (1.32, 1.49, and 1.29 microgram/L), two of which also differed significantly in accuracy. The 86Rb assay measured glycoside activity corresponding to a mean digoxin concentration of 1.35 microgram/L. We conclude that the relatively low precision of digoxin assay and the variations in accuracy between kits from various vendors apparently deserve continual attention.
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