A solid-spiking matrix matched calibration strategy for simultaneous determination of cadmium and chromium in sediments by isotope dilution laser ablation inductively coupled plasma mass spectrometry

New quantification of Cd and Cr in sediments by isotope dilution LA-ICPMS, significantly simplifies and shortens the isotope dilution step.

The Perennial Problem of Variability In Adenosine Triphosphate (ATP) Tests for Hygiene Monitoring Within Healthcare Settings

OBJECTIVETo investigate the reliability of commercial ATP bioluminometers and to document precision and variability measurements using known and quantitated standard materials.METHODSFour commercially branded ATP bioluminometers and their consumables were subjected to a series of controlled studies with quantitated materials in multiple repetitions of dilution series. The individual dilutions were applied directly to ATP swabs. To assess precision and reproducibility, each dilution step was tested in triplicate or quadruplicate and the RLU reading from each test point was recorded. Results across the multiple dilution series were normalized using the coefficient of variation.RESULTSThe results for pure ATP and bacterial ATP from suspensions ofStaphylococcus epidermidisandPseudomonas aeruginosaare presented graphically. The data indicate that precision and reproducibility are poor across all brands tested. Standard deviation was as high as 50% of the mean for all brands, and in the field users are not provided any indication of this level of imprecision.CONCLUSIONSThe variability of commercial ATP bioluminometers and their consumables is unacceptably high with the current technical configuration. The advantage of speed of response is undermined by instrument imprecision expressed in the numerical scale of relative light units (RLU).Infect Control Hosp Epidemiol2015;00(0):1–6

EUCAST Testing of Isavuconazole Susceptibility in Aspergillus: Comparison of Results for Inoculum Standardization Using Conidium Counting versus Optical Density

ABSTRACTThe EUCAST E.DEF9.1 standard recommends standardization of the inoculum concentration by conidium counting using a hemocytometer rather than a spectrophotometer. In this study, we investigated whether the choice of these methods influenced isavuconazole MICs. A blinded collection of 30 molecularly characterized azole-resistant isolates and 10 wild-typeAspergillus fumigatusisolates was shared with four different laboratories. Additionally, each laboratory selected approximately 100A. fumigatusisolates and 50 isolates each ofA. flavus,A. nidulans,A. niger, andA. terreus(1,237 isolates in total). Three laboratories (laboratories 1 to 3) used conidium counting. One laboratory standardized the inoculum using a spectrophotometer (that is, by use of the optical density [OD]) and is referred to as the OD laboratory. Correlation coefficients, intraclass correlation coefficients, and essential agreement were calculated, and 2-log-unit differences were assessed (pairedttest). The MIC range for the blinded collection was 0.25 to 16 mg/liter, and a 1-dilution-step difference between the MIC50and MIC90across the four laboratories was detected and a 2-dilution-step difference between the modal MICs was detected. Compared to the results for laboratories 1 and 2, a significant correlation was found for the OD laboratory MIC data (correlation coefficients, 0.85 and 0.93, respectively; intraclass correlation coefficients, 0.88 and 0.96, respectively). The number of mutant isolates whose MICs overlapped those of the wild-type isolates was the lowest for the OD laboratory (14/30 [46.7%] mutant isolates), whereas the numbers were 18/30 (60%) isolates for laboratory 1, 17/30 (56.7%) isolates for laboratory 2, and 21/30 (70%) isolates for laboratory 3. For theA. flavus,A. fumigatus,A. nidulans,A. niger, andA. terreusisolates, comparative analysis again defined the MIC distributions from the OD laboratory to be in excellent agreement with those from laboratories 1 and 2 across all fiveAspergillusspp. The findings suggest that EUCAST testing using OD determination is an appropriate alternative for standardization ofAspergillusinoculum concentrations.

Comparative Analysis of the Susceptibility to Triclosan and Three Other Biocides of Avian Salmonella enterica Isolates Collected 1979 through 1994 and 2004 through 2010

Few studies have been conducted on changes in the susceptibility of bacteria due to long-term use of biocides. A total of 375 avian Salmonella isolates collected in Germany from healthy or diseased animals during two time periods, 1979 through 1994 and 2004 through 2010, were included in the present study. The isolates were tested for their MICs of triclosan, acriflavine, benzalkonium chloride, and chlorhexidine by broth macrodilution. MIC50, MIC90, and the distribution of MICs were compared. The MIC ranges were 0.0625 to 0.5 μg/ml for triclosan, 16 to 256 μg/ml for acriflavine, 8 to 128 μg/ml for benzalkonium chloride, and 0.5 to 32 μg/ml for chlorhexidine. MIC50s and MIC90s were equal or differed by not more than one dilution step. For isolates from healthy poultry collected during the two time periods, statistical analysis revealed a significant increase only in MICs for chlorhexidine. Salmonellae from diseased birds were more susceptible to triclosan and benzalkonium chloride but less susceptible to acriflavine and chlorhexidine. Overall, only 25 strains had the highest detected MIC of 0.5 μg/ml triclosan, but an association with multidrug resistance could not be confirmed.

New Statistical Technique for Analyzing MIC-Based Susceptibility Data

ABSTRACTSeventeen laboratories participated in a cooperative study to validate the regional susceptibility testing ofNeisseria gonorrhoeaein The Netherlands. International reference strains were distributed. Each laboratory determined the MICs of ciprofloxacin, penicillin, and tetracycline, for each strain by Etest. To explore a more transparent assessment of quality and comparability, a statistical regression model was fitted to the data that accounted for the censoring of the MICs. The mean MICs found by all of the laboratories except three were closer than one 2-fold dilution step to the overall mean, and the mean MICs of each antimicrobial agent were close to the MICs for the international reference strains. This approach provided an efficient tool to analyze the performance of the Dutch decentralized gonococcal resistance monitoring system and confirmed good and comparable standards.

Pseudohyponatremia Revisited: A Modern-Day Pitfall

Factitiously low sodium estimations are a hazard in most modern clinical laboratories. Most modern high-throughput analyzers use indirect ion-selective electrodes to estimate electrolyte concentrations in serum samples. This analysis is preceded by a dilution step of the sample. If the water concentration is altered by the presence of increased lipid or protein, the dilution step and the subsequent calculation of concentration by the analyzer results in a falsely low sodium value. This places patients at risk, particularly if the factitious result is acted upon by the physician. In this short review, we highlight this problem and review the methodology and situations where this artifact can occur and discuss strategies to circumvent this problem. When factitious results are suspected, whole blood sodium can be assessed using a direct ion-selective electrode, by measurement of osmolality, or by calculation of the serum water fraction and applying a correction to the reported value.

In Vitro Antifungal Susceptibilities and Amplified Fragment Length Polymorphism Genotyping of a Worldwide Collection of 350 Clinical, Veterinary, and Environmental Cryptococcus gattii Isolates

ABSTRACT The in vitro susceptibilities of a worldwide collection of 350 Cryptococcus gattii isolates to seven antifungal drugs, including the new triazole isavuconazole, were tested. With amplified fragment length polymorphism (AFLP) fingerprinting, human, veterinary, and environmental C. gattii isolates were subdivided into seven AFLP genotypes, including the interspecies hybrids AFLP8 and AFLP9. The majority of clinical isolates (n = 215) comprised genotypes AFLP4 (n = 76) and AFLP6 (n = 103). The clinical AFLP6 isolates had significantly higher geometric mean MICs for flucytosine and fluconazole than the clinical AFLP4 isolates. Of the seven antifungal compounds examined in this study, isavuconazole had the lowest MIC90 (0.125 μg/ml) for all C. gattii isolates, followed by a 1 log2 dilution step increase (MIC90, 0.25 μg/ml) for itraconazole, voriconazole, and posaconazole. Amphotericin B had an acceptable MIC90 of 0.5 μg/ml, but fluconazole and flucytosine had relatively high MIC90s of 8 μg/ml.

Establishing Quality Assurance Criteria for Serial Dilution Operations on Liquid-Handling Equipment

Since the advent of high-throughput screening (HTS) in the early 1990s, parallel multichannel liquid handlers have become a mainstay in every drug discovery setting. Although several peer-reviewed publications have discussed methods and criteria for stamping multiwell copies, there is very little information about establishing a standard operating procedure (SOP) for standard (microliter-level) serial dilutions of compounds used in dose-response experiments. The authors discuss the 4 main criteria any serial dilution process must pass (accuracy, precision, fold dilution, and outliers) and the process for establishing thresholds for all of these values in a compound management or biological screening laboratory. The thresholds need to be both low enough to be acceptable from a biological potency variability perspective and high enough to allow the instruments to pass the quality assurance (QA) analysis on a regular basis. In this article, the authors suggest suitable thresholds arrived at by a variety of methods, including trend analysis of QA data, survey questionnaire from the main stakeholders (screening scientists, chemists), and published criteria for single-shot stamping. A mathematical analysis of the effect of threshold values on estimated XC50s was performed to ensure that the variability introduced by the serial dilution step is within acceptable overall variability limits.

Determining Dilution Accuracy in Microtiter Plate Assays Using a Quantitative Dual-Wavelength Absorbance Method

Dilution-based volume transfer steps are commonly performed for many types of microtiter plate-based assays. Because the exact concentrations of target samples in solution are typically not directly measured but calculated from the dilution ratio, it is important to accurately measure each dilution step so that the ultimate concentrations of those samples can be known. This paper describes a new approach, which is based on dual-dye, dual-wavelength photometry, for accurately measuring each dilution step in a multistep dilution protocol. The theory behind this method is described, as well as an experimental demonstration of the prescribed approach.