[Errors and faulty calculations in medicobiological research. III. Sources of error in chemical analysis].
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Chemistry Techniques, Analytical↗
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Potential sources of errors in cation-exchange chromatographic measurement of plasma taurine.
We examined the potential sources of error in automated cation-exchange chromatographic quantitation of plasma taurine, both in sample preparation and in the analysis. Principal sources of error include: use of serum instead of plasma, which produces gross overestimates; use of tripotassium ethylenediaminetetraacetate (EDTA) as anticoagulant in systems involving ninhydrin detection (a ninhydrin-positive contaminant of EDTA emerges coincident with taurine); contamination with platelets; and placing volumes exceeding 20 microL on the cartridge used in the Technicon TSM Amino Acid Analyzer. We arrived at a simple technique in which we use EDTA as anticoagulant, micropore filtration to produce platelet-free plasma, and o-phthalaldehyde as the detection reagent for the sensitivity required to measure accurately the low concentration of taurine in plasma.
Autoanalysis↗
Abstinence from smoking and expired-air carbon monoxide levels: lactose intolerance as a possible source of error.
We investigated a possible source of contamination of expired-air carbon monoxide (CO) readings on CO monitors from raised expired-air hydrogen (H2) levels, namely lactose intolerance, the norm in certain ethnic groups. A significant correlation between expired-air CO and H2 readings was observed in four non-smoking lactose maldigesters.
Breath Tests↗
Fracture stiffness measurement using the orthometer: reproducibility and sources of error.
OBJECTIVES: To elicit the reproducibility and the common sources of error in the use of the Orthometer, a commercially available goniometer based system, designed to measure the stiffness of healing fractures. DESIGN: A laboratory based study. BACKGROUND: The Orthometer is widely used to measure the progress of fracture healing in a quantitative manner. It has been shown previously that the bending stiffness of a fracture increases with healing and that a stiffness of 15 Nm/degree equates with the functional union of a tibial fracture. METHODS: The Orthometer was attached in a standard manner to nylon bars of known stiffness to determine the accuracy of the device. The Orthometer was then set up with changes in a single positional variable to assess the effect of this variable on the measurement accuracy. A number of different clinicians were asked to use the Orthometer before and after a simple training session and any improvements in the measurement accuracy were observed. RESULTS: Stiffness could be measured to within 10% or less of the true stiffness. Markedly different degrees of error were introduced with the various set-up variables. A simple training session improved clinician accuracy. CONCLUSIONS: Although the Orthometer does allow some degree of safety margin, it is essential that set-up is performed carefully, and that the clinician has been trained in the use of the device. RELEVANCE: Quantitative measurements of fracture healing have enormous potential benefits over manual and radiological methods of assessments. Provided these are carefully performed, the accuracy of these measurements is good.
Elasticity↗
Use of visual perception in estimating static postural stresses: magnitudes and sources of errors.
Very little is known about the magnitudes and sources of errors associated with the visual estimation of postural classification displayed on TV screens. This study was conducted to address this issue. Sixty-three subjects participated in the experiments. The findings indicate that: (1) subjects found it difficult to evaluate upper extremity postures (particularly the elbow and the wrist), while the postures around the lower back were the easiest to evaluate; (2) the lower extremity positions affected the ability of the subjects to accurately classify postures around the wrist, elbow, shoulder, neck, and lower back, with the estimates being > 70% for sitting and > 60% for standing (except for the elbow); and (3) in general, flexion and extension are easier to evaluate than neutral and non-neutral postures.
Adult↗
Optical radiation measurements: instrumentation and sources of error.
Accurate measurement of optical radiation is required when sources of this radiation are used in biological research. The most difficult measurements of broadband noncoherent optical radiations usually must be performed by a highly trained specialist using sophisticated, complex, and expensive instruments. Presentation of the results of such measurement requires correct use of quantities and units with which many biological researchers are unfamiliar. The measurement process, physical quantities and units, measurement systems with instruments, and sources of error and uncertainties associated with optical radiation measurements are reviewed.
Optics and Photonics↗
Sample filtration as a source of error in the determination of trace metals in marine waters.
Adequate performance in interlaboratory proficiency tests using filtered, pre-treated or pre-digested test materials does not necessarily demonstrate that laboratories' data are of adequate comparability. Sample handling can be an important source of error which is not examined by routine proficiency tests. This paper reports a study of sample filtration as a source error in the determination of trace metals in marine water samples. The results indicate that that current practice may need to be reviewed if important contamination errors are to be controlled.
Equipment Contamination↗
CT and MRI derived source localization error in a custom prostate phantom using automated image coregistration.
Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique. Additionally, accurate dosimetry may be useful in predicting the success of an implant. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. MRI is recognized as a superior imaging modality in delineating the prostate gland. More importantly, MRI can be used for source localization in postimplant prostates. However, the MRI derived source localization error bears further investigation. We present a useful tool in determining the source localization error as well as permitting the fusion, or coregistration, of selected data from multiple imaging modalities. We constructed a custom prostate phantom of hydrocolloid material precisely implanted with I-125 seeds. We obtained CT, the accepted modality, and MRI scans of the phantom. Subsequently, we developed an automated algorithm that employs a sequential translation of data sets to initially maximize coregistration and minimize error between data sets. This was followed by a noniterative solution for the necessary rotation transformation matrix using the Orthogonal Procrustes Solution. We applied this algorithm to CT and MRI scans of the custom phantom. CT derived source locations had source localization errors of 1.59 mm +/- 0.64. MRI derived source locations produced similar results (1.67 mm +/- 0.76). These errors may be attributed to the image digitization process.
Algorithms↗
Assessing the communication skills of doctors in training: reliability and sources of error.
Medical examining bodies now commonly assess candidates' communication skills. However, there are a number of within-case sources of error that can mean examinations have poor reliability and validity. The aims of this study were to determine the main within-case sources of error and to identify the best methods of maximising reliability in highly structured communication skills assessments. Subjects were fifth year medical students at the University of Aberdeen. Subjects were videotaped giving clinical information to standardised patients and relatives toward the end of their fifth year and toward the end of their pre-registration year. Sources of variation, reliability coefficients and the most effective methods of reducing measurement error were calculated using generalisability theory. Systematic differences among subjects' communication skill was the main source of variation in two of the three assessment cases. The implications of the results are discussed.
Communication↗
Sources of error in intraocular lens power calculation.
The hypothesis that the minimum error in predicted refraction after implantation of an intraocular lens (IOL) of calculated power is the sum of the random error in (1) the measurement of the axial length, (2) the measurement of the corneal power, and (3) the estimation of the pseudophakic anterior chamber depth (ACD) is proposed. Based on preoperative and postoperative biometry of 584 IOL implantations, 54% of the error was attributed to axial length errors, 8% to corneal power errors, and 38% to errors in the estimation of the postoperative ACD, when a fixed ACD was used in the IOL calculations. However, if the ACD was predicted according to a previously described regression method, the contribution of error from the ACD source was reduced to 22%, thereby reducing the total refractive prediction error from +/- 1.03 diopters (D) (+/- SD) to +/- 0.92 D (+/- SD). These predictions accord with clinical results.
Adult↗
Cell adherence as a serious source of error in the haemacytometer count of leucocytes in artificial suspensions.
This report demonstrates a previously disregarded source of error in the count of lymphocytes that have been fixed in Turk's dilution fluid (acetic acid and gentian violet). If the dilution fluid does not contain any serum protein or other proteins, a large proporation of the lymphocytes will stick to the walls of the mixing vessel. The use of plastic or siliconized vessels does not prevent this. The number of cells which in that way will be eliminated from the suspension is dependent on several variables, such as the type of mixing procedure and mixing vessel, the compensation of the dilution fluid and the cell density. As a consequence it is difficult to give any correction factor for such a count in a haemacytometer counting chamber. More than 30% of the cells may disappear under generally obtained conditions. This source of error is to some extent also involved in the white blood cell count, despite the presence of about 1% protein in the dilution fluid from the serum and lysed erythrocytes. To prevent this loss of cells either bone serum albumin, to a concentration of 5%, or the detergent Certrimide, to a concentration of 2-5 mM, may be added to the dilution fluid.
Animals↗
The effects of measurement errors in the plasma radioactivity curve on parameter estimation in positron emission tomography.
The effects of three error sources in plasma curve measurements on parameter estimation in kinetic analysis of positron emission tomography (PET) fluorodeoxyglucose (FDG) data are investigated by computer simulation. The three error sources are: (1) measurement noise in the radioactivity concentrations of plasma samples; (2) linear interpolation between adjacent plasma sampling points of the plasma time activity curve; and (3) incorrect weights used for the least-squares regression. All three error sources are found to increase the variability of the parameter estimates, with the first one a primary error source in normal PET FDG studies. The performance of five estimation methods which account for the error sources are evaluated. When the noise variances of the plasma and the tissue measurements are not known, an iterative weighting procedure is shown to give accurate and reliable estimates.
Computer Simulation↗
Sources of error in electronic structure calculations on small chemical systems.
The sources of error in electronic structure calculations arising from the truncation of the one-particle and n-particle expansions are examined with very large correlation consistent basis sets, in some cases up through valence 10-zeta quality, and coupled cluster methods, up through connected quadruple excitations. A limited number of full configuration interaction corrections are also considered. For cases where full configuration interaction calculations were unavailable or prohibitively expensive, a continued fraction approximation was used. In addition, errors arising from corevalence and relativistic corrections are also probed for a number of small chemical systems. The accuracies of several formulas for estimating total energies and atomization energies in the complete basis set limit are compared in light of the present large basis set findings. In agreement with previous work, the CCSD(T) method is found to provide results that are closer to the CCSDTQ and full configuration-interaction results than the less approximate CCSDT method.
Journal Article↗
System performance evaluation of refractive surgical lasers: a mathematical approach.
A study was conducted for the purpose of improving the designs of the next generation of refractive surgical laser systems. Two common refractive laser systems, variable-spot scanning (type A) and small-spot scanning (type B), are discussed by identifying sources of error that could adversely affect the capability of these lasers to accurately produce complex, customized wavefront guided ablations. A mathematical model was used to construct a laser simulator that models the two common laser systems in terms of the root-mean-square error. Error sources from ablation profile fitting, ablation registration, eye tracking, and the laser delivery system are compared, and the relative contribution of each to the overall system error is analyzed. This system-level analysis can be helpful to the improvement of both laser systems.
Computer Simulation↗
Sources of error with use of Goldmann-type tonometers.
There are many sources of error in the use of Goldmann-type applanation tonometers. In clinically normal corneas hypofluorescence of the precorneal tear film, accommodation, the Valsalva maneuver and vertical gaze are preventable causes of large tonometric errors. Repeated tonometry may induce a decline in the intraocular pressure (IOP). Variations in the corneal resistance to indentation between eyes cause significant errors. The most significant cause of error in clinically abnormal eyes is corneal epithelial edema, which causes a marked underestimation of IOP. Measurements obtained with Goldmann-type tonometers can be used with confidence to monitor changes in the IOP of an individual, but should not be relied on to determine the absolute manometric pressure within an eye or to compare the IOPs in eyes of different individuals.
Contact Lenses↗
Endoluminal ultrasography for rectal tumors: efficacy, sources of error and limitations.
OBJECTIVES: endorectal ultrasound (EUS) is currently accepted as the best technique for the preoperative study of patients with rectal tumors, and surgical decisions depend increasingly on EUS staging. The main pitfalls in staging rectal tumors are over- or understaging as well as errors in imaging lymph nodes. Being aware of such errors and their causes may help to improve the overall results. The aim of the present study was to evaluate the accuracy of EUS in staging rectal neoplasms, and to study potential sources of error. METHODS: from May 1996 to December 1998, 120 patients with rectal tumors were studied preoperatively by EUS. The uTNM classification described by Hildebrandt and coworkers was used. The EUS findings were compared prospectively with the results of pathological examination. When there was no correlation, both the specimen and the EUS findings were carefully reviewed to look for potential sources of error. RESULTS: 41 out of 120 patients were classified as uT1, 10 as uT2, 60 as uT3 and 9 out of 120 as uT4. 31 patients had positive lymph nodes (uN1). On comparing these data with the results of the pathological report, we found 90% accuracy in staging rectal wall penetration, and 70% accuracy in the diagnosis of lymph nodes. Errors were due basically to technical problems, characteristics of the tumor itself, and difficulties in staging lymph nodes. CONCLUSIONS: it is important to identify the potential source of errors as well as the current limitations of EUS to improve the overall results with this technique.
Adult↗
An approach to validation of a leg simulation by the comparison of two dynamic models.
The dynamics of a jointed leg were simulated using two different models, one based on a recursive Newton-Euler method and one on a closed-form Lagrange method. To validate the models, the simulations were run in parallel and the intermediate steps and output of the two methods were compared to one another to reveal the presence and locations of errors. Sources of error and the use of this method for the detection of errors are discussed. Some errors could not have been detected using only a single simulation.
Acceleration↗
Questions on validity of International Classification of Diseases-coded diagnoses.
International Classification of Diseases (ICD) codes are used for indexing medical diagnoses for various purposes and in various contexts. According to the literature and our personal experience, the validity of the coded information is unsatisfactory in general, however the 'correctness' is purpose and environment dependent. For detecting potential error sources, this paper gives a general framework of the coding process. The key elements of this framework are: (1) the formulation of the established diagnoses in medical language; (2) the induction from diagnoses to diseases; (3) indexing the diseases to ICD categories; (4) labelling of the coded entries (e.g. principal disease, complications, etc.). Each step is a potential source of errors. The most typical types of error are: (1) overlooking of diagnoses; (2) incorrect or skipped induction; (3) indexing errors; (4) violation of ICD rules and external regulations. The main reasons of the errors are the physician's errors in the primary documentation, the insufficient knowledge of the encoders (different steps of the coding process require different kind of knowledge), the internal inconsistency of the ICD, and some psychological factors. Computer systems can facilitate the coding process, but attention has to be paid to the entire coding process, not only to the indexing phase.
Abstracting and Indexing↗