Optimization of the decision threshold for single radioactive counting

When the activity of a sample is close to the background level, the decision threshold is classically defined by considering distributions of sample and background as being equal. Recently, the Bayesian approach has been considered in the standard ISO to refine the determination of the decision threshold by taking into account all accessible information prior to measurement such as type A and type B uncertainties. However, simplifications using Gaussian approximation and experimental values instead of true means are often used to facilitate calculations. In this paper, we develop a complete treatment without simplification, based on the Bayesian approach and Poisson distribution. Minimal informations have been considered: one single raw counting for the sample and one previously acquired background. From one single background counting, the net background probability law is calculated and then a decision threshold is deduced. In particular, we demonstrate that the decision threshold is defined for any case including very low background or even null event. Comparisons with classical approach as well as the Bayesian treatment in the new ISO 11929 have been carried out. Applications of this decision threshold for the optimisation of radioactive measurement or in case of a set of minimal detectable activities used to determine average releases are given. Bayesian treatment also gives relevant informations such as the probability for a source to be radioactive when the net number of counts is below the decision threshold.

Bacterial growth kinetics: modelling and evaluation of two-compartment radioassay

Quantitative measurements of bacterial growth may be made using a radioassay technique. This method measures, by scintillation counting, the 14CO2 derived from die bacterial metabolism of a 14C-labeled substrate. Mathematical growth models may serve as reliable tools for estimation of the generation rate constant (or slope of the growth curve) and provide a basis for evaluating assay performance. Two models, i.e., exponential and logistic, are proposed. Both models yielded an accurate fit to the data from radioactive measurement of bacterial growth. The exponential model yielded high precision values of the generation rate constant, with an average relative standard deviation of 1.2%. Under most conditions the assay demonstrated no changes in the slopes of growth curves when the number of bacteria per inoculation was changed. However, the radiometric assay by scintillation method had a growth-inhibiting effect on a few strains of bacteria. The source of this problem was thought to be hypersensitivity to trace amounts of toluene remaining on the detector.Key words: bacterial growth modelling.

IODOTHYRONINE CONTENT OF HUMAN THYROID ALBUMIN

ABSTRACT Stable thyroid hormones (T4 and T3)1) have been demonstrated in pure albumin isolated from normal human thyroid tissue iodinated in vivo. Five samples of albumin were separated from other thyroid proteins by acrylamide gel electrophoresis. After pronase hydrolysis, the content of Thyroid hormones was measured chemically (T4 + T3) as well as by competitive radioactive measurement (T4) and radioimmunoassay (T3). The purity of the albumin and validity of these measurements were confirmed by different techniques. The synthesis of thyroid hormones is not therefore a property unique to Tg and may occur in albumin. However the amount of iodothyronines in the albumin (average 0.004 residue per molecule) is much less than that found in Tg (0.5 residue per molecule). In the albumin as in Tg the number of hormone residues per molecule is proportional to the number of atoms of iodine. At an equivalent iodine concentration, the albumin seems capable of forming the thyroid hormones as well as Tg. The difference between these two proteins, in their capacity to synthesize thyroid hormones, seems to depend on their capacity for iodination. This difference of iodination does not seem to be linked with the number of tyrosyl residues, but might be related to the position of these residues.