The interlaboratory performance of microbiological methods for food analysis.

Repeatability and reproducibility data for microbiological methods in food analysis were collated and assessed with a view to identifying useful or important trends. Generalized additive modeling for location, shape, and scale was used to model the distribution of variances. It was found that mean reproducibility for log10(CFU) data is largely independent of concentration, while repeatability SD of log10(CFU) data shows a strongly significant decrease in repeatability SD with increasing enumeration. The model for reproducibility SD gave a mean of 0.44, with an upper 95th percentile of approximately 0.76. Repeatability variance could be described reasonably well by a simple dichotomous model; at enumerations below 10(5)/g, the model for repeatability SD gave a mean of approximately 0.35 and upper 95th percentile of 0.63. Above 10(5)/g, the model gave a mean of 0.2 and upper 95th percentile of 0.36. A Horwitz-like function showed no appreciable advantage in describing the data set and gave apparently worse fit. The relationship between repeatability and reproducibility of log10(CFU) is not constant across the concentration range studied. Both repeatability and reproducibility were found to depend on matrix class and organism.

Scoring in genetically modified organism proficiency tests based on log-transformed results.

The study considers data from 2 UK-based proficiency schemes and includes data from a total of 29 rounds and 43 test materials over a period of 3 years. The results from the 2 schemes are similar and reinforce each other. The amplification process used in quantitative polymerase chain reaction determinations predicts a mixture of normal, binomial, and lognormal distributions dominated by the latter 2. As predicted, the study results consistently follow a positively skewed distribution. Log-transformation prior to calculating z-scores is effective in establishing near-symmetric distributions that are sufficiently close to normal to justify interpretation on the basis of the normal distribution.

Absolute bioavailability of [14C] genistein in the rat; plasma pharmacokinetics of parent compound, genistein glucuronide and total radioactivity.

The systemic plasma pharmacokinetics of genistein were determined in rats to evaluate the absolute oral bioavailability and make comparison with similar data in the literature derived from humans subjects. The plasma concentrations of genistein, genistein glucuronide and carbon-14 were determined by LC-MS/MS and liquid scintillation counting following oral and intravenous dosing with [14C]genistein (4 mg kg(-1) body weight). The absorption of total radioactivity from the gut, (parent compound and metabolites), was 56 and 111% in male and female rats, respectively. In contrast, the absolute oral bioavailability of genistein in male and female rats was 7 and 15%. There was a significant (P<0.001) difference between Cmax of genistein after intravenous (6921 and 4392 ng/ml) and oral (21 and 22 ng/ml) dosing in male and female rats, respectively. After oral administration, the concentration profile of genistein glucuronide in plasma greatly exceeded that of parent compound during the absorption/distribution phase suggesting extensive first pass metabolism, and provided evidence of entero-hepatic circulation. Selective plasma analysis by LC-MS/MS, without prior enzymatic hydrolysis, enabled ready discrimination between parent and conjugated metabolites and prevented gross overestimation of genistein bioavailability. Pharmacokinetic parameters Cmax, Tmax and AUC were similar to those reported in humans, which supports the use of the rat model for genistein toxicity studies.

Precision estimates produced by specially-designed ruggedness tests compared with those derived from collaborative trials, in relation to estimation of measurement uncertainty.

We have compared the reproducibility precisions provided by collaborative trials with precisions obtained from ruggedness tests specially designed to try to simulate between-laboratory variation. We found that the ruggedness tests underestimated the reproducibility variability consistently for those analytical methods requiring empirical calibration. The ruggedness tests provided on average a precision merely comparable with repeatability precision. This finding has implications for the estimation of uncertainty.