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Biomedical subjects

W A Sadler

Publications and source records attributed to W A Sadler.

At least 19 recordsLinked to original sources
PubMed2002

A new WIN32 computer program for estimating immunoassay variance functions.

A new WIN32 computer program for estimating immunoassay variance functions is described. It replaces and enhances an MS-DOS version written more than a decade ago. Input data can consist of either runs of raw replicated measurements (maximum: 40,000 observations) or sets of values of N, mean and SD (maximum: 20,000 sets). Data can be imported from Paradox, dBase, Foxpro or Access database tables, from text files, or via the Windows clipboard. The program produces histogram plots of sets of replicates and plots containing up to 12 variance functions expressed in terms of variance, SD or CV(%) versus the mean (imprecision profiles). Any specified view of a variance function graph can be inset into any other view. Interactive point and click design dialogues give virtually complete control over graph appearance. Graphs can be exported to external applications via the clipboard or disk files, in either bitmap or Windows enhanced metafile format.

Analysis of Variance↗
PubMed1997

A pragmatic approach to estimating total analytical error of immunoassays.

Several groups have recently commented on the need for more realistic information on analytical performance of laboratory tests. The term "total analytical error" is sometimes used in this context. However, differing opinions have been expressed on how best to obtain estimates of clinical assay error, as it would be perceived by clinicians. We suggest a pragmatic definition of total analytical error for immunoassays and describe our attempts to estimate it by simple designs in the internal quality-control process. We use results over 29 months from a total serum thyroxine RIA. The most important error sources were those related to calibration materials and operator effects, errors not usually captured by short-term or snapshot experiments.

Humans↗
PubMed1996

Influence of specimen carryover on sensitive thyrotropin (TSH) assays: is there a problem?

A relatively slow transition has occurred from so-called 1st-generation thyrotropin (TSH) assays (e.g., RIAs) through 2nd-generation assays (e.g., IRMAs) to 3rd-generation assays (e.g., immunochemiluminometric assays). Analysis of data from a modified internal quality-control design, followed up by a computer simulation, showed that specimen carryover has minimal effect on 2nd-generation TSH assays. However, extension of the simulation to a 3rd-generation assay showed the possibility of substantial effects in the subnormal region. Carryover of 1:1250 (0.08%), for example, may reduce the theoretical 10-fold precision improvement claimed for 3rd-generation assays to nearer fourfold. Simulation results suggest maximum allowable specimen carryover of approximately 1:10,000 (approximately 0.01%) for 3rd-generation TSH assays. We suggest that when automated specimen handling is used in a TSH assay, a well-designed carryover experiment should become a routine part of reports that claim 3rd-generation (or better) performance characteristics.

Autoanalysis↗
PubMed1992

Factors affecting bone mineral density in high school girls.

OBJECTIVES: first to establish a local normal range for hip and spine bone density in the teenage years. Secondly to determine what factors might affect bone mineral density at this age. METHODS: bone mineral density (DPX absorptiometer) at the hip and spine in a cohort of 138 high school girls; mean age 16.4 yr (SD 0.34). Anthropometric factors, calcium intake, physical activity and other lifestyle and medical data were documented in each subject. RESULTS: in this group of 16 year old schoolgirls mean bone mineral density at the hip, 1.01 (0.13) was not significantly different from 20-25 year old New Zealand females, but bone mineral density at lumbar spine, 1.17 (0.12), was significantly lower. Positive correlations of bone mineral density with weight, height, physical activity and calcium intake were demonstrated. Weight was clearly the best predictor of bone mineral density variability. Calcium intake and physical activity showed no predictive value at the spine but contributed significantly at all regions of the femur and particularly at the trochanter. CONCLUSIONS: it appears that peak bone mass can be modified by nutrition and exercise. Adolescents should be encouraged into regular exercise programmes and to maintain adequate body mass and calcium intakes.

Absorptiometry, Photon↗
PubMed1992

Minimum distinguishable difference in concentration: a clinically oriented translation of assay precision summaries.

A trend to market-driven health care in many parts of the world is focusing increasing attention on getting maximum value from available resources. Laboratories are not exempted. Well-informed clinical input has a potentially valuable role in any laboratory rationalization process. However, a communication difficulty exists in the sense that, although laboratory workers, commercial developers, regulatory bodies, etc., are thoroughly conditioned to using assay coefficient of variation as a general performance measure (for excellent reasons), this is not necessarily the most intuitive or informative scale from a clinician's perspective. Here we use routine clinical data from an immunoradiometric assay of thyrotropin to illustrate, first, a general approach to estimation and prediction of reproducibility, and second, an alternative summary that expresses the discriminatory power of an assay. This latter measure, our experience suggests, is more suited to the way clinicians perceive assays and assay results. The overall aim is improved clinician/laboratory communication.

Biological Assay↗
PubMed1990

A computer program for estimating imprecision characteristics of immunoassays.

A reliable numerical algorithm is described, together with a computer program written in FORTRAN IV and FORTRAN 77, for estimating a three-parameter variance function by approximate conditional likelihood. The function is sufficiently flexible to provide for a several thousand-fold relative change in variance and appears to be a good model for the severely heteroscedastic results obtained from immunoassays. The computer program is primarily intended for summarizing imprecision characteristics of immunoassays in the form of imprecision profiles, but the estimated variance functions have additional application whenever further parametric analysis of immunoassay results is undertaken (e.g., as a weighting function when immunoassay results are used in a least-squares regression analysis). The flexibility of the function implies useful application in any area where heteroscedasticity is particularly severe.

Algorithms↗
PubMed1990

Use and abuse of imprecision profiles: some pitfalls illustrated by computing and plotting confidence intervals.

Published estimates of the imprecision characteristics of immunoassays are often based on quantities of data that seem to be inadequate. The increasing use of imprecision profiles has not necessarily improved the situation. We describe and illustrate a method of computing 95% confidence intervals for profiles estimated directly from replicated assay results. The data used were chosen to mimic the sort of data that are typically available in practice, either within laboratories or from external quality-assessment programs. Use of confidence intervals is an effective pictorial way of incorporating information about the quantity and the distribution of the data used for estimation. This is in keeping with an important property of imprecision profile plots: potentially complex information is summarized in a readily comprehensible way. A computer program is available for estimating and plotting profiles and their 95% confidence intervals.

Computer Simulation↗
PubMed1988

The quality of performance of the fructosamine test.

The performance of fructosamine assays in 35 laboratories was assessed over a six month period in 1987. While agreement between laboratories was poor in the range expected in diabetic patients, most laboratories were internally consistent. The interlaboratory differences are largely attributed to the calibration methods used for this test. Fructosamine results from any one laboratory must not be interpreted by comparison with data from other laboratories or the literature. Other common biochemical tests were found to suffer from similar problems, but to a less serious extent.

Calibration↗
PubMed1988

A method for direct estimation of imprecision profiles, with reference to immunoassay data.

A three-parameter model for directly estimating imprecision profiles from replicated immunoassay results is compared with a six-parameter indirect profile model obtained by the usual method of combining error in the raw response measurements with the slope of the standard curve. Direct estimation is likely to be less reliable when based on the limited data collected from a single assay, and may underestimate variability at high concentrations when many results are clustered at the upper end of the concentration range. However, at concentrations near the assay detection limit (often a region of particular interest), direct estimation is superior to the indirect method if a logistic or related function is used as the standard curve model. Direct estimation of imprecision profiles has useful application whenever the internal details of an assay system are not readily available, for example, in analysis of data collected in external surveys.

Computer Simulation↗
PubMed1987

A computer program for variance function estimation, with particular reference to immunoassay data.

A computer program is described for estimating a simple one or two parameter variance function using approximate conditional likelihood. The inverse of the fitted function provides a smooth weighting function for use in subsequent manipulation of the data. The program is designed for immunoassays, where both the raw experimental observations, and the final results from data reduction, are well known to exhibit nonuniformity of variance, but it may also have application in other areas where the observations follow approximately normal distributions and a weighting function is required. Five variance function forms are optionally available as subroutines, and if necessary, one or more of these can be incorporated into existing software with trivial modification. The test of fit of the assumed function provides an objective criterion for detecting outliers, and the program includes automatic outlier exclusion as an option.

Algorithms↗
PubMed1987

A reliable and inexpensive microprocessor system for collection and transfer of raw immunoassay data.

A very low cost microprocessor system has been designed to ease data handling problems in a large workload immunoassay laboratory. The microprocessor collects and stores data from many immunoassay detection devices simultaneously, and transfers the data to a minicomputer for analysis as each measurement batch is completed. Stored data are protected against a mains power failure during collection and against non-availability of the minicomputer at transfer time. The system provides fast and reliable transfer of very large amounts of raw data from measurement devices to computer, and therefore facilitates the use of a statistically sound data reduction software package.

Computers↗
PubMed1987

Estimation of imprecision in immunoassay quality assessment programmes.

A computer was used to produce and store a large body of simulated thyroxine and thyrotropin radioimmunoassay results over wide concentration ranges. Sets of 24 results, designed to represent results returned to an external quality assessment survey, were randomly drawn from the stored results in order to compare limited data survey estimates of imprecision with the 'true' values obtained from the population data. Results show that estimation of an imprecision profile has advantages over the usual index of imprecision, but that neither form of estimate is particularly reliable when the available data is limited. Results suggest that survey organisers should place considerably less emphasis on results from any one survey and greater emphasis on analysing cumulative data and method comparison data.

Computer Simulation↗