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D M Giltinan

Publications and source records attributed to D M Giltinan.

10 recordsLinked to original sources
PubMed1996

The effect of variance function estimation on nonlinear calibration inference in immunoassay data.

Often with data from immunoassays, the concentration-response relationship is nonlinear and intra-assay response variance is heterogeneous. Estimation of the standard curve is usually based on a nonlinear heteroscedastic regression model for concentration-response, where variance is modeled as a function of mean response and additional variance parameters. This paper discusses calibration inference for immunoassay data which exhibit this nonlinear heteroscedastic mean-variance relationship. An assessment of the effect of variance function estimation in three types of approximate large-sample confidence intervals for unknown concentrations is given by theoretical and empirical investigation and application to two examples. A major finding is that the accuracy of such calibration intervals depends critically on the nature of response variance and the quality with which variance parameters are estimated.

Algorithms↗
PubMed1994

Assays for recombinant proteins: a problem in non-linear calibration.

Quantification of protein levels in biological matrices such as serum or plasma frequently relies on the techniques of immunoassay or bioassay. The relevant statistical problem is that of non-linear calibration, where one estimates analyte concentration in an unknown sample from a calibration curve fit to known standard concentrations. This paper discusses a general framework for calibration curve fit to known standard concentrations. This paper discusses a general framework for calibration inference, that of the non-linear mixed effects model. Within this framework, we consider two issues in depth: accurate characterization of intra-assay variation, and the use of empirical Bayes methods in calibration. We show that proper characterization of intra-assay variability requires pooling of information across several assay runs. Simulation work indicates that use of empirical Bayes methods may afford considerable gain in efficiency; one must weigh this gain against practical considerations in the implementation of Bayesian techniques. We illustrate the methods discussed using a cell-based bioassay for the recombinant hormone relaxin.

Bayes Theorem↗
PubMed1994

Effect of environment on insulin-like growth factor I refolding selectivity.

Recombinant human insulin-like growth factor I (IGF-I), a 70-amino-acid peptide containing three disulphide bonds, produces two monomeric and several multimeric species during refolding. To optimize production of correctly folded IGF-I, conditions which influence protein refolding, stability and solubility were systematically examined. Combinations of solution components and conditions were analysed to identify synergistic interactions which enhance or reduce refolding efficiency. IGF-I concentration had the largest effect on formation of correctly folded peptide, due to competing association reactions. Solution polarity had the next largest individual effect, both on the level of multimeric peptide and on the relative proportion of correctly folded to misfolded monomeric peptide. Salt type and concentration and chaotrope type and concentration also had large individual effects on the distribution of IGF-I forms produced. Solution polarity modulated the effects of many other conditions including chaotrope concentration, salt type and concentration, and osmolyte concentration. Simultaneously decreasing the solution polarity and increasing the salt concentration improved the yield of correctly folded IGF-I relative to either individual change. Optimum solution conditions for refolding were 2M urea, 1M NaCl and 20% (v/v) ethanol. A possible mechanism for the importance of solution polarity on IGF-I refolding is discussed.

Amino Acid Sequence↗
PubMed1993

Some general estimation methods for nonlinear mixed-effects models.

A nonlinear mixed-effects model suitable for characterizing repeated measurement data is described. The model allows dependence of random coefficients on covariate information and accommodates general specifications of a common intraindividual covariance structure, such as models for variance within individuals that depend on individual mean response and autocorrelation. Two classes of procedures for estimation in this model are described, which incorporate estimation of unknown parameters in the assumed intraindividual covariance structure. The procedures are straightforward to implement using standard statistical software. The techniques are illustrated by examples in growth analysis and assay development.

Calibration↗
PubMed1989

Fitting heteroscedastic regression models to individual pharmacokinetic data using standard statistical software.

In the analysis of individual pharmacokinetic data by nonlinear regression it is important to allow for possible heterogeneity of variance in the response. Two common methods of doing this are weighted least squares with appropriate weights or data transformation using a suitable transform. With either approach it is appealing to let the data determine the appropriate choice of weighting scheme or transformation. This article describes two methods of doing this which are easy to compute using standard statistical software. The first method is a generalized least squares scheme for the case where the variance is assumed proportional to an unknown power of the mean. The second involves applying a power transformation to both sides of the regression equation. It is shown that both techniques may be implemented using only nonlinear regression routines. Sample code is provided for their implementation using the SAS software package. However, the proposed methods are feasible using any software package that incorporates a nonlinear least squares routine, and are thus well suited to routine use.

Humans↗
PubMed1987

Effects of bifonazole, fluconazole, itraconazole, and terbinafine on the chemiluminescence response of immune cells.

The luminol-enhanced chemiluminescence (CL) assay was used to examine the effects of antifungal agents tested at concentrations above and below therapeutically achievable levels on the CL response of mouse spleen cells. Reduction in the CL response of phagocytic cells may be indicative of an inhibition of the cellular immune response. Concomitantly, an increase in the CL response of phagocytic cells may indicate an enhancement of the immune capacity of these cells. The effects of four antifungal agents, bifonazole, fluconazole (UK-49,858), itraconazole, and terbinafine were studied. Changes in the CL response were assessed in terms of peak intensity, time to peak intensity, and area under the intensity-time curve compared with appropriate diluent controls for each drug. Both bifonazole and itraconazole caused significant reduction in peak CL intensity only at the highest level assayed (20 mg/l). Fluconazole had no significant effect on the CL response of mouse spleen cells at levels up to 20 mg/l, inclusive. Although terbinafine had no significant effect on peak CL intensity, it did cause a significant decrease in time to peak response at levels above 5 mg/l. This decrease in time to peak response may be indicative of an enhancement in the immune capacity of the mouse spleen cells; the clinical significance of this observation remains to be determined.

Animals↗
PubMed1987

Candida tropicalis infection in normal, diabetic, and neutropenic mice.

Opportunistic infections caused by Candida tropicalis have been noted with increasing frequency in compromised patients. The pathogenicity of three isolates of C. tropicalis was studied in normal CD-1 mice, streptozotocin-induced diabetic mice, and cyclophosphamide-induced neutropenic mice. Lethal dose 50% endpoints and quantitative distribution of yeast cells in spleen, liver, and kidneys of mice infected intravenously were determined. The virulence of one yeast isolate was greater than that of the other two. The order of susceptibility to mortality and degree of organ colonization was neutropenic greater than diabetic greater than normal. Renal lesions resembling those associated with infection by C. albicans appeared by day 5 postinfection in diabetic and neutropenic mice. Greater numbers of C. tropicalis yeast cells were isolated from homogenates of the affected kidneys, suggesting that the kidney is a target organ for this fungus. This study demonstrates the increased susceptibility of compromised mice to C. tropicalis as compared with normal mice and verifies the ability of these yeasts to cause opportunistic disease.

Agranulocytosis↗
PubMed1986

Influence of six antifungal agents on the chemiluminescence response of mouse spleen cells.

The luminol-enhanced chemiluminescence (CL) assay is a measure of the early events of phagocytosis, leukocyte activation, and immune cell interactions. Reduction in the CL response of immune cells may be indicative of an inhibition of the immune response. This study was undertaken to examine the effects of antifungal agents tested at concentrations above and below therapeutically achievable levels on the CL response of mouse spleen cells. The effects of six antifungal agents--amphotericin B, ketoconazole, miconazole, 5-fluorocytosine, Bay-n-7133, and ICI 153,066--were studied. Changes in the CL response were assessed in terms of peak intensity and area under the intensity-time curve compared with appropriate diluent controls for each drug. Only amphotericin B and ketoconazole caused statistically significant lowering of the peak intensity at levels that are therapeutically attainable (mean peak plasma levels of 2 to 4 and 3.5 to 16 micrograms/ml, respectively). Although amphotericin B and ketoconazole caused reduction in the CL response, removal of the unbound drug in the preparation by centrifugation, washing, and suspension of cells in fresh drug-free medium resulted in a return of CL activity equivalent to the activity observed in cells not exposed to these agents. These results suggest that amphotericin B and ketoconazole at therapeutic concentrations may cause a reduction in immune cell antimicrobial activity; the clinical significance of these observations remains to be determined.

Amphotericin B↗
PubMed1986

Design and analysis considerations in evaluating the chemiluminescence response of mouse spleen cells.

Chemiluminescence is the result of the respiratory burst generated by phagocytic cells after stimulation by antigen. The measurement of chemiluminescence represents a sensitive means for detecting antigenic stimulation and immune cell function. Although the kinetics of chemiluminescence reactions have been described, appropriate statistical methods for the evaluation of data from chemiluminescence assays have not been reported. Based on examination of data from several studies in which the chemiluminescence response of spleen cells was investigated after stimulation with the particulate antigen zymosan, recommendations are made for the design and statistical evaluation of such studies. Three parameters were used in assessing the chemiluminescence response; peak intensity of the emitted light, time to peak, and the area under the intensity-time curve. The data indicated that peak intensity alone provides an adequate characterization of the chemiluminescence response. Since percent change in response upon treatment is of interest, analysis on the log scale is appropriate, and the statistical procedure of choice in evaluating data of this type is a trend analysis. The need for a balanced allocation of treatments to avoid potential bias is demonstrated. The methods proposed are illustrated with data from two studies in which the effect of preincubation with low concentrations of ketoconazole, an antifungal agent, on the chemiluminescence response of BALB/cBY spleen cells was examined.

Analysis of Variance↗