SEARCH · PubMed Health

Results for “Uncertainty Quantification”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources

PubMed2006

Application of Poisson kriging to the mapping of cholera and dysentery incidence in an endemic area of Bangladesh.

BACKGROUND: Disease maps can serve to display incidence rates geographically, to inform on public health provision about the success or failure of interventions, and to make hypothesis or to provide evidences concerning disease etiology. Poisson kriging was recently introduced to filter the noise attached to rates recorded over sparsely populated administrative units. Its benefit over simple population-weighted averages and empirical Bayesian smoothers was demonstrated by simulation studies using county-level cancer mortality rates. This paper presents the first application of Poisson kriging to the spatial interpolation of local disease rates, resulting in continuous maps of disease rate estimates and the associated prediction variance. The methodology is illustrated using cholera and dysentery data collected in a cholera endemic area (Matlab) of Bangladesh. RESULTS: The spatial analysis was confined to patrilineally-related clusters of households, known as baris, located within 9 kilometers from the Matlab hospital to avoid underestimating the risk of disease incidence, since patients far away from the medical facilities are less likely to travel. Semivariogram models reveal a range of autocorrelation of 1.1 km for dysentery and 0.37 km for cholera. This result translates into a cholera risk map that is patchier than the dysentery map that shows a large zone of high incidence in the south-central part of the study area, which is quasi-urban. On both maps, lower risk values are found in the Northern part of the study area, which is also the most distant from the Matlab hospital. The weaker spatial continuity of cholera versus dysentery incidence rates resulted in larger kriging variance across the study area. CONCLUSION: The approach presented in this paper enables researchers to incorporate the pattern of spatial dependence of incidence rates into the mapping of risk values and the quantification of the associated uncertainty. Differences in spatial patterns, in particular the range of spatial autocorrelation, reflect differences in the mode of transmission of cholera and dysentery. Our risk maps for cholera and dysentery incidences should help identifying putative factors of increased disease incidence, leading to more effective prevention and remedial actions in endemic areas.

Bangladesh↗

PubMed2001

Expression of results with uncertainty for the determination of pesticides in melon--experience in a proficiency test.

The expression of results with an uncertainty through the "bottom-up" approach, involving the estimation and combination of all the sources of uncertainty, represents a challenge when the analytical method includes mass transfer steps (MTS). These steps (e.g. extraction, evaporation, digestion, etc.) with inherently different from 100% recoveries lack models capable of describing their precision and efficiency. Recently, a new methodology was published aimed at the estimation of the performance of these critical steps. Comparison of the experimental dispersion from the replicated analysis of spiked samples with the combination of the uncertainty associated with gravimetric, volumetric and instrumental quantification steps (described by well established models) allows the estimation of the MTS uncertainty. Evaluation of the behaviour of the MTS within the analytical range supports the use of developed estimations over a wide concentration range. This methodology was applied, with success, to the determination of pesticide residues in melon in one particular proficiency test organised by the Food Analysis Performance Assessment Scheme (FAPAS) between November 2000 and February 2001.

Chromatography, Gas↗

PubMed1995

Non-T1-weighted 31P chemical shift imaging of the human liver.

A 31P chemical shift imaging (CSI) protocol was developed for human liver studies. It is shown that at the commonly used repetition time (TR) of 1 s T1-weighting reduces the integrated intensities of liver phosphate metabolite signals by 18 +/- 15% (inorganic phosphate, Pi) to 46 +/- 10% (phosphodiester, PDE), that is for an RF pulse angle of 60 degrees (weighted average) in liver. The loss in signal-to-noise ratio (S/N) at TR = 20 s, sufficient to eliminate spectral distortions caused by saturation, compared with TR = 1 s (47-65%) can be overcome by using one-dimensional (1D)-phase encoding with a small number of phase-encode steps. The liver spectra obtained by 1D-CSI with 4-step phase-encoding (spatial resolution 10 cm) have the highest S/N and, after multiplication of the PDE signal by a factor of 1.4, closely reflect the liver metabolite levels. It is concluded that clinical 31P MR studies of liver function can be performed without T1-weighting and that the current practice to compromise the MRS quantification of lever metabolites with uncertainties caused by (differential changes in) T1-weighting is not warranted.

Humans↗

PubMed2005

Chromatographic preprocessing of GC-MS data for analysis of complex chemical mixtures.

Hyphenated analytical techniques such as gas chromatography-mass spectrometry (GC-MS) can provide extensive amounts of analytical data when applied to environmental samples. Quantitative analyses of complex contaminant mixtures by commercial preprocessing software are time-consuming, and baseline distortion and incomplete peak resolution increase the uncertainty and subjectivity of peak quantification. Here, we present a semi-automatic method developed specific for processing complex first-order chromatographic data (e.g. selected ion monitoring in GC-MS) prior to chemometric data analysis. Chromatograms are converted into semi-quantitative variables (e.g. diagnostic ratios (DRs)) that can be exported directly to appropriate softwares. The method is based on automatic peak matching, initial parameterization, alternating background noise reduction and peak estimation using mathematical functions (Gaussian and exponential-Gaussian hybrid) with few (i.e. three to four) parameters. It is capable of resolving convoluted peaks, and the exponential-Gaussian hybrid improves the description of asymmetric peaks (i.e. fronting and tailing). The optimal data preprocessing suggested in this article consists of estimation of Gaussian peak parameters and subsequent calculation of diagnostic ratios from peak heights. We tested the method on chromatographic data from 20 replicate oil samples and found it to be less time-consuming and subjective than commercial software, and with comparable data quality.

Gas Chromatography-Mass Spectrometry↗

PubMed2000

Methods for ELNES-quantification: characterization of the degree of inversion of Mg-Al-spinels

The energy loss near-edge structures in electron energy-loss spectra contain information about bonding characteristics, the electronic structure and coordinations of the excited atoms. We have calculated sets of reference spectra for the normal and for the inverse Mg-Al-spinel using a full multiple scattering approach. By a quantitative comparison of these reference spectra with experimental data ELNES-quantification becomes possible. We characterized the degree of inversion lambda by the analysis of the relative peak-intensities and the relative peak-positions within 35 eV beyond the edge onset. The results demonstrate that by using the provided methods ELNES-quantification will become possible when uncertainties in the experiment are reduced and a better fit of the simulations to the experiment is achieved.

Journal Article↗

PubMed2000

Direct in situ measurements of labile inorganic and organically bound metal species in synthetic solutions and natural waters using diffusive gradients in thin films.

The emerging technique of DGT (diffusive gradients in thin films) is shown to be capable of performing new speciation measurements in situ in natural waters. In DGT, metals are bound to a resin layer after passing through a well-defined diffusion layer. Cd was measured in solutions containing glycine, EDTA, and fulvic (FA) and humic acids (HA) by atomic absorption spectroscopy (AAS), anodic stripping voltammetry (ASV), and DGT. DGT measured similar labile fractions to ASV, with detailed differences being consistent with a thicker diffusion layer allowing more dissociation of labile complexes and a slower diffusion of FA and HA complexes through the gel. When single measurements are made in complex solutions with DGT, precise quantification is impossible due to uncertainties concerning the distribution of species with different diffusion coefficients. A new procedure was proposed based on the advantage of DGT of being able to control the pore size of the diffusive gel layer. Small (inorganic) species diffuse freely through all gels but larger FA and HA (organic) complexes diffuse less freely in more constrained gels. When measurements were made on known solutions of Cu and FA or HA, it was possible to quantify the inorganic and organic species separately. They agreed well with predictions made using the WHAM speciation code. Multiple DGT units were also deployed in situ in a stream with high dissolved organic carbon (14.6 mg/L). The systematic differences between the devices with different gel compositions enabled determination, for the first time, of the in situ concentrations of both labile inorganic and organic species in natural water. A single DGT device with a constrained gel can be used to quantify inorganic species directly, providing absolute accuracy is not required. This ability of DGT to measure well-defined fractions of metals in situ using a simple device gives it considerable potential as a regulatory tool. A direct speciation measurement may be preferable to modeling approaches which require diverse input data that are difficult to determine.

Benzopyrans↗

PubMed2003

[Genotyping and RNA quantification of hepatitis C virus. ].

Genotyping and quantification of hepatitis C virus is of importance in the assessment of prognosis and choice of treatment. We present the results from a study of 144 patients over two years in our hospital and point out the degree of uncertainty involved in the method of quantification. Doctors treating patients with hepatitis C should know the level of precision of the applied method.

Genotype↗

PubMed2006

[Quantification of prostate movements during radiotherapy].

Decrease treatment uncertainties is one of the most important challenge in radiation oncology. Numerous techniques are available to quantify prostate motion and visualise prostate location day after day before each irradiation: CT-scan, cone-beam-CT-Scan, ultrason, prostatic markers... The knowledge of prostate motion is necessary to define the minimal margin around the target volume needed to avoid mispositioning during treatment session. Different kind of prostate movement have been studied and are reported in the present work: namely, those having a large amplitude extending through out the whole treatment period on one hand; and those with a shorter amplitude happening during treatment session one the other hand. The long lasting movement are mostly anterior-posterior (3 mm standard deviation), secondary in cranial-caudal (1-2 mm standard deviation) and lateral directions (0.5-1 mm standard deviation). They are mostly due to the rectal state of filling and mildly due to bladder filling or inferior limbs position. On the other hand, the shorter movement that occurs during the treatment session is mostly variation of position around a steady point represented by the apex. Ones again, the rectal filling state is the principle cause. This way, during the 20 minutes of a treatment session, including the positioning of the patient, a movement of less than 3 mm could be expected when the rectum is empty. Ideally, real time imaging tools should allow an accurate localisation of the prostate and the adaptation of the dosimetry before each treatment session in a time envelope not exceeding 20 minutes.

Environmental Monitoring↗

PubMed2003

Spaceborne estimate of atmospheric CO2 column by use of the differential absorption method: error analysis.

For better knowledge of the carbon cycle, there is a need for spaceborne measurements of atmospheric CO2 concentration. Because the gradients are relatively small, the accuracy requirements are better than 1%. We analyze the feasibility of a CO2-weighted-column estimate, using the differential absorption technique, from high-resolution spectroscopic measurements in the 1.6- and 2-microm CO2 absorption bands. Several sources of uncertainty that can be neglected for other gases with less stringent accuracy requirements need to be assessed. We attempt a quantification of errors due to the radiometric noise, uncertainties in the temperature, humidity and surface pressure uncertainty, spectroscopic coefficients, and atmospheric scattering. Atmospheric scattering is the major source of error [5 parts per 10 (ppm) for a subvisual cirrus cloud with an assumed optical thickness of 0.03], and additional research is needed to properly assess the accuracy of correction methods. Spectroscopic data are currently a major source of uncertainty but can be improved with specific ground-based sunphotometry measurements. The other sources of error amount to several ppm, which is less than, but close to, the accuracy requirements. Fortunately, these errors are mostly random and will therefore be reduced by proper averaging.

Journal Article↗

PubMed2004

[Cost-utility analysis; uncertainties restrict applicability].

Assumptions and choices are inherent to cost-utility analysis, the economic technique that provides a generic measure for the quantification of the efficiency of various health-care services. Uncertainty prevails on many of them: on the definition of the output of health care as health gain, on its expression in quality-adjusted life years (QALYs), on the choice of measurement instruments that should provide comparable information across all disease categories, on the choice of the procedure to value health states, on the relative importance of the judgments of patients and the general public, on the quantification of various types of costs, and on the question as to whether, and then how, future health gains should be discounted. The conclusion is that the results of a cost-utility analysis should be interpreted with caution. For the time being, cost-utility analysis cannot be used for direct comparisons of efficiency across all types of health-care services.

Attitude to Health↗

PubMedDate not supplied

Processing biological images from very low light emissions.

As opposed to strong light signals (> 10(8) photons mm-2 s-1), the construction of images from object sources with low level signals (< 10(2) photons mm-2 s-1) involves a probabilistic transfer- or point-spread function. The resulting images carry considerable uncertainty or spread, restricting resolution and quantification. In this paper we propose various strategies how to reconstruct object characteristics from very low light emissions by unfolding the imaging equation. Further, calibration techniques which help to associate light emissions with the decomposition of luminogenic substrates in a spatially selective way will be discussed.

Animals↗

PubMed2003

Modelling biological nutrient removal activated sludge systems - a review.

The external nitrification (EN) biological nutrient removal (BNR) activated sludge (ENBNRAS) system shows considerable promise for full-scale implementation. As an aid for this implementation, a mathematical simulation model would be an invaluable tool. To develop such a model, a study was conducted to select the most suitable simulation model to serve as a starting point for further development. For this, the existing available simulation models for BNRAS systems are compared with one another and evaluated against experimental observations in the literature and on ENBNRAS systems. One process immediately apparent to be crucially important is the anoxic growth of phosphorus accumulating organisms (PAOs), with associated PAO denitrification and anoxic P uptake for polyP formation. These linked processes are lacking in the earlier kinetic simulation models for BNRAS systems, which were based on aerobic PAO growth and P uptake only, but have been incorporated into the more recent kinetic models. This provides a substantive body of information on modelling this aspect. Other processes of significance identified to require consideration are anaerobic slowly biodegradable COD (SBCOD) hydrolysis to readily biodegradable COD (RBCOD), and COD loss. Both processes have significant impact on the predicted BEPR performance. Due to the uncertainties associated with the mechanisms and quantification of these two processes, it is concluded that the most extensively validated kinetic simulation model should be selected for development, and that the omissions in this model should be addressed progressively, using the relevant information drawn from the existing models, the literature and observations on ENBNRAS systems.

Bioreactors↗

PubMed2002

The sequestration switch: removing industrial CO2 by direct ocean absorption.

This review paper considers direct injection of industrial CO2 emissions into the mid-water oceanic column below 500 m depth. Such a process is a potential candidate for switching atmospheric carbon emissions directly to long term sequestration, thereby relieving the intermediate atmospheric burden. Given sufficient research justification, the argument is that harmful impact in both the Atmosphere and the biologically rich upper marine layer could be reduced. The paper aims to estimate the role that active intervention, through direct ocean CO2 storage, could play and to outline further research and assessment for the strategy to be a viable option for climate change mitigation. The attractiveness of direct ocean injection lies in its bypassing of the Atmosphere and upper marine region, its relative permanence, its practicability using existing technologies and its quantification. The difficulties relate to the uncertainty of some fundamental scientific issues, such as plume dynamics, lowered pH of the exposed waters and associated ecological impact, the significant energy penalty associated with the necessary engineering plant and the uncertain costs. Moreover, there are considerable uncertainties regarding related international marine law. Development of the process would require acceptance of the evidence for climate change, strict requirements for large industrial consumers of fossil fuel to reduce CO2 emissions into the Atmosphere and scientific evidence for the overall beneficial impact of ocean sequestration.

Absorption↗

PubMed1993

The evaluation and interpretation of cervical cytology: application of the likelihood ratio concept.

The Papanicolaou smear (Pap test), used for the detection and prevention of neoplastic lesions of the cervix, is known to have both false negative and false positive results. Proper handling of the diagnostic uncertainty resulting from these errors demands quantification of flaws. Traditionally, sensitivity, specificity and predictive values are used for that aim. In this study another approach is advocated, namely the use of the likelihood ratio. For cervical cytology this ratio is the quotient of the probability of a Pap class within the diseased population to the probability of that same Pap class within the non-diseased group. This approach enables the characterization of each Pap class separately, and is therefore much better for clinical interpretation of the result. It is also a superior approach for quality assessment.

Female↗

PubMed2005

Simultaneous analysis of twenty-one glucocorticoids in equine plasma by liquid chromatography/tandem mass spectrometry.

A method for the simultaneous separation, identification, quantification and confirmation of the presence of 21 glucocorticoids (GCC) in equine plasma by liquid chromatography coupled with triple stage quadrupole tandem mass spectrometry (LC/TSQ-MS/MS) is described. Plasma sample augmented with the 21 GCC was extracted with methyl tert-butyl ether (MTBE) and analyzed by positive electrospray ionization. Desoxymetasone or dichlorisone acetate was used as the internal standard (IS). Quantification was performed by IS calibration. For each drug, one major product ion was chosen and used for screening for that drug. Analyte confirmation was performed by using the three most intense product ions formed from the precursor ion and the corresponding mass ratios. The recovery of the 21 GCC when spiked into blank plasma at 5 ng/mL was 45-200% with coefficient of variation (CV) from 0.3-18%. The limit of detection (LOD) and that of quantification (LOQ) for most of the analytes were 50-100 pg/mL and 1 ng/mL, respectively, whereas that of confirmation (LOC) was 100-300 pg/mL depending on the analyte. Intra- and inter-day precisions expressed as CV for quantification of 1 and 10 ng/mL was 1.0-17%, and 0.51-19%, respectively, and the accuracy was from 84-110%. The linear concentration range for quantification was 0.1-100 ng/mL (r(2) > 0.997). Estimated measurement uncertainty was from 11-37%. This study was undertaken to develop a method for simultaneous screening, identification, quantification and confirmation of these agents in post-race equine plasma samples. The method has been successfully applied to screening of a large number of plasma samples obtained from racehorses in competition and in pharmacokinetic studies of dexamethasone in the horse and concurrent changes in endogenous GCC, hydrocortisone and cortisone. The method is simple, sensitive, selective and reliably reproducible.

Animals↗

PubMed2005

The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars.

Some heavy elements (with atomic number A > 69) are produced by the 'rapid' (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from approximately 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way, to determine the U/Th production ratio very precisely (0.57(+0.037)(-0.031). This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations, to help determine the source of Galactic cosmic rays, and to date circumstellar grains. I also estimate the age of the Milky Way (14.5(+2.8)(-2.2)Gyr in a way that is independent of the uncertainties associated with fluctuations in the microwave background or models of stellar evolution.

Journal Article↗

PubMed2005

Development of a liquid chromatography/tandem mass spectrometry method for the quantification of fumonisin B1, B2 and B3 in cornflakes.

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the determination of fumonisin B1 (FB1), B2 (FB2) and B3 (FB3) in cornflakes is described. During method development, special attention was paid to the selection of a suitable internal standard (IS) in order to offer a good alternative for deuterated FB1. In this respect, the C12-sphinganine analogue (2S,3R)-2-aminododecane-1,3-diol was chosen because of its structural similarity to the fumonisin backbone and its chromatographic elution between the target analytes. For the extraction of the fumonisins from the cornflakes matrix, MeOH/H2O (adjusted to pH 4 with 0.1 M HCl; 70:30, v/v), ACN/MeOH/H(2)O (25:25:50, v/v/v) and acidified ACN/MeOH/H2O (25:25:50, v/v/v; pH 4) were evaluated. Preference was given to acidified MeOH/H2O (70:30, v/v) with mean recoveries (n=12) for FB1, FB2 and FB3 of, respectively, 84+/-10, 78+/-7 and 85+/-9%. Cleanup was performed using immunoaffinity columns (FumoniTest, VICAM). The chromatography was performed under isocratic conditions at a flow of 0.3 mL min-1 with a mobile phase consisting of ACN/H2O (60:40, v/v) containing 0.3% formic acid. The mass spectrometer was operated in the positive electrospray ionization (ESI+) mode using multiple reaction monitoring (MRM). An intralaboratory validation was conducted with fortified samples determining limits of detection (LOD), limits of quantification (LOQ), precision, trueness, specificity and measurement uncertainty. The LOD concentrations for FB1, FB2 and FB3 were 20, 7.5 and 12.5 microg/kg. The LOQs were 40 microg/kg for FB1, 15 microg/kg for FB2 and 25 microg/kg for FB3. The coefficients of variation (CVs) under repeatability conditions varied from 11 to 13% for FB1, from 9 to 14% for FB2 and from 7 to 10% for FB3. Under within-laboratory reproducibility conditions, the CVs ranged from 12 to 17% for FB1, from 9 to 16% for FB2 and from 7 to 13% for FB3. The percent bias for FB1 varied from -12 to -10%, while for FB2 and FB3 bias ranged, respectively, from -4 to -2% and from -12 to -5%. The expanded measurement uncertainties for FB1, FB2 and FB3 were, respectively, 19, 18 and 22%.

Chromatography, Liquid↗

PubMed2004

Controlled serial grinding for high-resolution three-dimensional reconstruction.

This work reports an improved preparation procedure for acquiring SEM images for three-dimensional (3D) reconstruction. The images are acquired consecutively after serial grinding and polishing. Ugelstad beads are used as landmarks for registration purposes and for determining the thickness of the abraded sections. An estimation of the bead sizes necessary for suitable thickness quantification at the corresponding spatial resolution and uncertainty is given. Shape-based interpolation is used for filling the gaps between the cross-sections. An indication of the distance between cross-sections necessary for good interpolation is also given. An example of a 3D reconstructed paper volume is presented. The method is suitable for preparation of fibre and paper as well as other materials.

Histocytological Preparation Techniques↗