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PubMedDate not supplied

A statistical overview on univariate calibration, inverse regression, and detection limits: Application to gas chromatography/mass spectrometry technique.

The paper summarizes critically the current approaches for the calculation of the limits of detection and quantification. In the context of the description of the method based on the calibration line, the arguments concerning the underlying experimental design, the choice of the appropriate model in the univariate regression, the effects of the dispersion characteristics of the data are deeply discussed. The effects of the scedasticity of the experimental data are taken into account in the obtainment of the calibration curve and in its utilization. To gain transparency, adaptability, and tutorial effectiveness the explicit formulas relevant to the use of straight line and quadratic models are reported. An application of the described procedures to GC-MS data is reported as an illustrative example.

Algorithms↗

PubMed2006

Sample size and statistical power assessing the effect of interventions in the context of mixture distributions with detection limits.

Often in randomized clinical trials and observational cohort studies, a non-negative continuously distributed response variable is measured in treatment and control groups. In the presence of true zeros for the response variable, a two-part zero-inflated log-normal model (which assumes that the data has a probability mass at zero and a continuous response for values greater than zero) is usually recommended. However, in some environmental health and human immunodeficiency virus (HIV) studies, quantitative assays for metabolites of toxicants, or quantitative HIV RNA measurements are subject to left-censoring due to values falling below the limit of detection (LD). Here, a zero-inflated log-normal mixture model is often suggested since true zeros are indistinguishable from left-censored values due to the LD. When the probabilities of true zeros in the two groups are not restricted to be equal, the information contributed by values falling below LD is used only to estimate the probability of true zeros in the context of mixture distributions. We derived the required sample size to assess the effect of a treatment in the context of mixture models with equal and unequal variances based on the left-truncated log-normal distribution. Methods for calculation of statistical power are also presented. We calculate the required sample size and power for a recent study estimating the effect of oltipraz on reducing urinary levels of the hydroxylated metabolite aflatoxin M(1) (AFM(1)) in a randomized, placebo-controlled, double-blind phase IIa chemoprevention trial in Qidong, China. A Monte Carlo simulation study is conducted to investigate the performance of the proposed methods.

Aflatoxin M1↗

PubMed2003

Relationship between detection limit and bias of accuracy of quantification of RNA by RT-PCR.

Evidence is presented demonstrating that the distribution of data obtained applying a given RT-PCR method deviates from a normal distribution depending on the limit of detection. The effect of this is a bias towards higher values and concomitantly a systematic error in respect to the accuracy of the evaluation due to this deviation from normality. In addition, evidence is presented that an evaluation assuming a log-normal distribution is more appropriate.

Data Interpretation, Statistical↗

PubMed2001

Picomolar detection limits with current-polarized Pb2+ ion-selective membranes.

Minor ion fluxes across ion-selective membranes bias submicromolar activity measurements with conventional ion-selective electrodes. When ion fluxes are balanced, the lower limit of detection is expected to be dramatically improved. As proof of principle, the flux of lead ions across an ETH 5435 ionophore-based lead-selective membrane was gradually compensated by applying a few nanoamperes of galvanostatic current. When the opposite ion fluxes were matched, and the undesirable leaching of primary ions was eliminated, Nernstian response down to 3 x 10(-12) M was achieved.

Journal Article↗

PubMed1999

Suppression of plasma virus load below the detection limit of a human immunodeficiency virus kit is associated with longer virologic response than suppression below the limit of quantitation.

Suppression of human immunodeficiency virus type 1 plasma virus load (PVL) to <20 copies/mL is associated with a longer virologic response after initiation of antiretroviral therapy. The relationship between duration of virologic response and PVL nadir according to a less sensitive assay was explored. When compared with subjects with a PVL nadir >500 copies/mL, the relative risks of PVL rising above 1000 copies/mL for participants in the INCAS trial and the British Columbia Drug Treatment Program with a PVL nadir below the limit of detection (LOD) were 0.04 (95% confidence interval [CI], 0.02-0.09) and 0.06 (95% CI, 0.03-0.12), respectively. The corresponding relative risks for persons with a detectable but not quantifiable PVL nadir were 0.25 (95% CI, 0.13-0.50) and 0.54 (95% CI, 0.25-1.19). The relative risks of virologic failure associated with a PVL nadir detectable but not quantifiable and a PVL nadir below the LOD were statistically different (P<.0001) in both data sets.

Acquired Immunodeficiency Syndrome↗

PubMed2001

Rotating electrode potentiometry: lowering the detection limits of nonequilibrium polyion-sensitive membrane electrodes.

A rotating electrode configuration is evaluated as a means to lower the detection limits of newly devised polyion-sensitive membrane electrodes (PSEs). Planar potentiometric polycation and polyanion PSEs are prepared by incorporating tridodecylmethylammonium chloride and calcium dinonylnaphthalenesulfonate, respectively, into plasticized PVC or polyurethane membranes and mounting disks of such films on an electrode body housed in a conventional rotating disk electrode apparatus. Rotation of the PSEs at 5000 rpm results in an enhancement in the detection limits toward heparin (polyanion) and protamine (polycation) of at least 1 order of magnitude (to 0.01 unit/mL for heparin; 0.02 microg/mL for protamine) over that observed when the EMF responses of the same electrodes are assessed using a stir-bar to achieve convective mass transport. A linear relationship between omega(-1/2), where omega is the rotating angular frequency, and C1/2, the polyion concentration corresponding to half the total maximum deltaEMF response toward the polyion species, is observed. It is further shown that the rotating polycation sensor can be used as an end-point detector to greatly enhance (relative to nonrotated indicator electrode) the analytical resolution and precision for measurement of low concentrations of heparin when such samples are titrated with protamine. The theoretical basis for lowering the detection limits by rotating PSEs is discussed based on the unique nonequilibrium response mechanism of such sensors.

Electrodes↗

PubMed2005

Assessing the effect of interventions in the context of mixture distributions with detection limits.

Many quantitative assay measurements of metabolites of environmental toxicants in clinical investigations are subject to left censoring due to values falling below assay detection limits. Moreover, when observations occur in both unexposed individuals and exposed individuals who reflect a mixture of two distributions due to differences in exposure, metabolism, response to intervention and other factors, the measurements of these biomarkers can be bimodally distributed with an extra spike below the limit of detection. Therefore, estimating the effect of interventions on these biomarkers becomes an important and challenging problem. In this article, we present maximum likelihood methods to estimate the effect of intervention in the context of mixture distributions when a large proportion of observations are below the limit of detection. The selection of the number of components of mixture distributions was carried out using both bootstrap-based and cross-validation-based information criterion. We illustrate our methods using data from a randomized clinical trial conducted in Qidong, People's Republic of China.

Anticarcinogenic Agents↗

PubMed2003

Detection limits of antimicrobials in ewe milk by delvotest photometric measurements.

The Delvotest method detection limits per manufacturer's instructions at a fixed reading time of 3 h for 24 antimicrobial agents were determined in ewe milk by photometric measurement. For each drug, eight concentrations were tested on 20 ewe milk samples from individual ewes. Detection limits, determined by means of logistic regression models, were (microg/kg): 3, amoxycillin; 2, ampicillin; 18, cloxacillin; 1, penicillin "G"; 34, cefadroxil; 430, cephalosporin "C"; 40, cephalexin; 20, cefoperazone; 33, Ceftiofur; 18, cefuroxime; 6100, streptomycin; 1200, gentamycin; 2600, neomycin; 830, erythromycin; 100, tylosin; 180, doxycycline; 320, oxytetracycline; 590, tetracycline; 88, sulfadiazine; 44, sulfamethoxazole; 140, sulfametoxypyridazine; 48, sulfaquinoxaline; 12,000, chloramphenicol; and 290, trimethoprim. Whereas the beta-lactam antibiotics, sulphonamides, and tylosin were detected by Delvotest method at levels equal to those of maximum residue limits, its sensitivity needs to be enhanced to detect aminoglycosides, tetracyclines, streptomycin, chloramphenicol, and trimethoprim residues in ewe milk or to develop an integrated residue detection system for ewe milk with different sensitive microorganisms for each group of antiinfectious agents.

Aminoglycosides↗

PubMed1987

Intracellular neutral carrier-based Ca2+ microelectrode with subnanomolar detection limit.

In intracellular electrolyte solutions a Ca2+-selective microelectrode based on the synthetic electrically neutral carrier N,N,N',N'-tetracyclohexyl-3-oxapentanediamide (ETH 129) shows an improved detection limit when compared with the so far widely used Ca2+ microelectrodes based on the neutral carrier ETH 1001. Detection limits are found at pCa = 9.2 in Ca2+ buffers containing an intracellular background of K+ (125 mM). Selectivity studies in mixed solutions show a preference of Ca2+ over Na+ of 6 X 10(5), over K+ of 1.6 X 10(6), and over Mg2+ of 5 X 10(6). The microelectrode does not suffer from significant interference by inorganic and organic inhibitors and by lipophilic cations and anions. The low detection limit is unchanged at least during the first eight hours of continuous contact with Ca2+ solutions. The EMF drift during the first hour of use is between 5 and 10 mV and is then reduced to about 1 mV/h. The changes in EMF induced between solution of pCa = 7 and pCa = 8 are reproducible within 24.7 +/- 0.4 mV (SD, n = 8, about 3 h). These electrode characteristics were found for single-barrelled microelectrodes of one micrometer diameter front-filled with a PVC-containing membrane phase. In the absence of poly(vinyl chloride) in the membrane phase irregular EMF response curves were obtained throughout. Preliminary punctures of ferret ventricular muscle cells indicate that the Ca2+ electrode response is not disturbed by the contact of a cytosolic milieu.

Animals↗

PubMed1994

Yoctomole detection limit by laser-induced fluorescence in capillary electrophoresis.

Laser-induced fluorescence detection in a sheath flow cuvette is the most sensitive method of detecting fluorescent molecules for capillary electrophoresis. This manuscript demonstrates the detection of 50 yoctomoles (1 yoctomole = 1 ymol = 1 x 10(-24) mol) of rhodamine 6G. A 1-mW He-Ne laser (lambda = 543.5 nm) was used as the excitation source. This detection limit is a 10 times improvement compared to the previous state-of-the-art detection limit in separation science.

Coloring Agents↗

PubMed2003

Determining "reliable detection limits" (RDLs) for environmental analyses.

It is generally accepted that the method detection limit or MDL (defined in 40 CFR 136, Appendix B) provides protection against false positives 99% of the time. This is correct, but only for the next single measurement performed after the MDL is determined. Subsequent measurements are not protected against false positives with the same degree of confidence, and there is no protection for false negatives. This paper provides a simple cost-effective approach for estimating the "reliable detection limit." Unlike the MDL, the statistic may be used for an indefinite number of future measurements and minimizes false negatives.

Environmental Monitoring↗

PubMed2005

Increase of the LC-MS/MS sensitivity and detection limits using on-line sample preparation with large volume plasma injection.

Large volume injection (LVI) has systematically been studied to improve LC-MS/MS sensitivity (signal-to-noise ratio, or S/N) and detection limits. The method of LVI was combined with on-line solid phase extraction (on-line SPE) and LC-MS/MS detection for analysis of compounds directly in plasma. It was demonstrated that LVI of plasma with on-line SPE-LC-MS/MS allows for improvement of sensitivity and detection limits without compromising chromatographic peak shape and resolution and inducing significant matrix and signal suppression effects. Furthermore, sensitivity and detection limits improve linearly with the injection volume up to 100 microL. Quantification of the model compounds in plasma demonstrated comparable calibration curve statistics, precision and accuracy for 5, 50 and 100 microL plasma injections.

Animals↗

PubMed1978

Experimental optimization of the detection limit of two-step solid-phase radioimmunoassay.

For two-step inhibition radioimmunoassay (sequential saturation or delayed addition of labeled antigen) generally a higher sensitivity than for one-step inhibition radioimmunoassay (equilibrium assay) is expected. The detection limit of a two-step solid phase inhibition radioimmunoassay for human serum immunoglobulin A was minimized by statistical methods of experimental optimization. Under optimal conditions the detection limit was 2.3 ng IgA. This is about 1.8 times lower than the minimal detection limit of the one-step assay under similar conditions of the qualitative variables such as the origin of the antibody. This increase in sensitivity was associated with a decrease in the precision of the assay. The results are discussed with respect to the comparison of the one-step and the two-step assay and the usefulness of a sensitive radioimmunoassay in practice.

Evaluation Studies as Topic↗

PubMed2005

Programmed potential sweep voltammetry for lower detection limits.

We report a novel programmed potential sweep voltammetry for a much lower detection limit than those achieved by any other known electroanalyitcal techniques. In this technique, an input waveform is programmed such that the background current would become flat or any other predefined form in the potential region of interest where the peak current arising from the analyte is observed, followed by the amplification of the background subtracted peak current. The current thus obtained showed a much better signal integrity at very low analyte concentrations than those obtained by the traditional linear sweep voltammetric and other related voltammetric techniques. The technique was applied to the analysis of dopamine at a carbon ultramicroelectrode (10-microm diameter). The background-compensated currents showed excellent dynamic linearity for dopamine concentrations of more than 3 orders of magnitudes between 500 pM and 100 nM with an estimated detection limit of 127 pM. This method can provide a convenient way for determining biogenic amines in real time with a much higher sensitivity.

Journal Article↗

PubMed1994

Combined use of 123I-labelled BCD-F9 and 4C4 monoclonal antibody with dissimilar specificity for breast cancer: implication for the detection limit of immunolymphoscintigraphy in the assessment of axillary lymph node metastases.

To determine whether the detection limit of immunolymphoscintigraphy (ILS), reported to be > or = 1 cm, can be improved by comparing imaging after administration of breast cancer-specific monoclonal antibody (MAb) BCD-F9 and breast cancer-nonspecific 4C4, 25 patients with suspected breast cancer were given injections of both 123I-labelled MAbs. The ILS was performed independently for both MAbs, the 4C4 scans serving as an ipsilateral negative control, and was used preoperatively to detect lymph node metastases. Twenty-one patients had breast cancer of whom 11 patients suffered from axillary involvement. Single interpretation of BCD-F9 scans gave true positive results in six of 11 and true negative results in 12 of 14 patients, whereas combined interpretation of BCD-F9 and 4C4 scans gave true positive results in nine of 11 and true negative results in 14 of 14 patients. On the basis of comparison of scintigrams of both MAbs, ILS allowed the detection of lymph node metastases 0.3-0.8 cm in diameter (n = 3). Immunohistochemistry of BCD-F9 and 4C4 MAbs of tumour-free and tumour-bearing lymph nodes correlated with ILS, with the exception of one patient. The study suggests that comparing scans obtained with BCD-F9 and 4C4 MAbs may improve the detection limit of ILS in the preoperative staging of axillae.

Axilla↗

PubMed2000

Improved detection limits and unbiased selectivity coefficients obtained by using ion-exchange resins in the inner reference solution of ion-selective polymeric membrane electrodes

By using a high concentration of an interfering ion and a low one of the primary ion in the inner reference solution of polymeric membrane ion-selective electrodes (ISEs), the lower detection limit may be improved and unbiased thermodynamic selectivity coefficients may be obtained. To this purpose, a cation-exchange resin is used here to keep the low concentration of the primary cation constant. Different compositions of the internal solution are required for obtaining optimal lower detection limits and unbiased selectivity coefficients. All ISEs studied here, i.e., for K+, Ca2+, and NH4+, based on valinomycin, ETH 5234, and nonactin/monactin, respectively, show improved lower detection limits in the range of 10(-7.6) (NH4+) to 10(-8.8) M (Ca2+). Nernstian responses and, therefore, unbiased selectivity coefficients are obtained with the K+-ISE for the discriminated ions, Na+, Mg2+, and Ca2+.

Journal Article↗

PubMed1995

Improved detection limit for catecholamines using liquid chromatography-electrochemistry with a carbon interdigitated array microelectrode.

The detection limit of catecholamines can be lowered by using a carbon-based interdigitated array (IDA) microelectrode as a detector for liquid chromatography (LC). The IDA electrode is more sensitive than conventional glassy carbon electrodes due to the high current density caused by radial diffusion at each microband, and redox cycling between two microband arrays. Since the number of redox cycles increases at lower flow-rates, the carbon IDA is particularly useful for microbore LC. In an LC system with a 1-mm microbore column and a carbon IDA electrode, the peak height of dopamine (DA) and DOPAC did not decrease with decreasing flow-rate because of this redox cycling. A low detection limit of 5 fg (32 amol) and 9.6 fg (57 amol) was obtained for DA and DOPAC due to the high current density and low background noise level (0.1 pA) at the carbon IDA electrode. The total charge generated by oxidizing DA at the anodic array was more than the value calculated by assuming that all the DA molecules were oxidized.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1991

Strategies for low detection limit measurements with cyclic voltammetry.

Cyclic voltammetry of Nafion-coated, carbon-fiber electrodes is used to detect trace concentrations of dopamine, both in a flow injection apparatus and in the brain of an anaesthetized rat. To improve signal-to-noise ratios, the sources of noise during cyclic voltammetry have been determined and strategies have been developed to decrease the noise. With the potentiostat employed, the measured noise is comparable to that expected for Johnson noise from the feedback resistor of the current transducer. Additional noise arises from the waveform generator employed and, in some cases, line noise. Line noise is discriminated against by starting each cyclic voltammogram either in phase or 180 degrees out of phase with the line frequency. When used in vivo, additional noise also arises from the physiological activity of the animal. Detection limits are found to closely correspond to those predicted on the basis of simulation of the voltammetric shape and the measured noise. Detection limits are improved by the use of appropriate analog and digital filtering, ensemble averaging, and appropriate timing of repetitive cyclic voltammograms. The combined use of these techniques enables the in vivo detection of approximately 100 nM of dopamine with a signal-to-noise ratio of 25.

Animals↗