Evaluation of systematic and random factors in measurements of fasting plasma glucose as the basis for analytical quality specifications in the diagnosis of diabetes. 3. Impact of the new WHO and ADA recommendations on diagnosis of diabetes mellitus

On behalf of the Danish Society of Clinical Endocrinology and the Danish Society of Clinical Chemistry we were commissioned to evaluate the influence of analytical and pre-analytical systematic and random factors on the diagnosis of diabetes, in order to provide a tool for conclusions on the analytical quality specifications needed to diagnose diabetes. A systems analysis was performed in accordance with the principles for evaluation of analytical quality specifications. The clinical setting was defined--diagnosis of diabetes in accordance with the WHO and ADA criteria with determination of fasting plasma glucose concentration (FPG) > or =7.0 mmol/L in two independent samples--with well-documented data on In (loge)-Gaussian distribution of reference values from a low-risk population and values for within-subject biological variation taken from the literature. An investigation was made of the consequences for the clinical setting of assumed errors related to the measurement of FPG. Four approaches were investigated for a single sampling and measurement and also for two independent samples: one showing the percentage of healthy individuals who had values > or = 7.0 mmol/L, one illustrating the origin of biological set-points for results > or = 7.0 mmol/L, one showing the risk of being measured > or =7.0 mmol/L when the biological set-point is known, and one showing the combined bias and imprecision for assumed percentages of false-positive (FP), defined as measurements > or = 7.0 mmol/L for the low-risk population and false-negative (FN), defined as measurements <6.4 mmol/L (the upper reference limit) for diabetics. This leaves a "grey zone" which includes the upper part of low-risk individuals, and defined by ADA and WHO as "impaired fasting glucose" (IFG). In the analysis, increasing systematic and random errors (combined analytical and pre-analytical) were assumed, and for each error condition the fractions of FP and FN were calculated. This gave plots from which the combined allowable systematic and random errors could be read off for pre-determined clinically acceptable fractions of FP and FN. The analysis does not distinguish between pre-analytical and analytical errors, as specified information on one of these is needed for specification of the other. The investigation provides a reliable basis for estimation of the needed analytical quality, and thereby for decisions about analytical quality specifications for analysis of FPG in relation to diagnosis of diabetes under optimized pre-analytical and analytical conditions. Consequences of deviations from these ideal conditions are illustrated in the figures, and should be considered for the different approaches with different performance conditions.     

Influence of index of individuality on false positives in repeated sampling from healthy individuals.

The index of individuality is defined as the ratio of the within-subject biological variation to the between-subject variation, i.e., the variation between the biological set-points. It has been disputed whether the index of individuality has influence on the usefulness of conventional population-based reference intervals. In this investigation we found that, as long as only a single sample is taken, for a certain change in an individual's set-point, the index of individuality has no influence on the usefulness of reference intervals. When two or more samples are taken into account, however, the outcome of the measurement is highly dependent on the index of individuality. For a low index, repeat measurement has only limited effect on the fraction of false-positive results, as the next result will be close to the first, but, when the index is high, the fraction of false-positive results will be reduced considerably through repeating the test. Moreover, the distribution of biological set-points for which the fraction of false-positive results originate is described and the influence of analytical imprecision is discussed. The calculations are performed for values of the index of individuality from 0 to 2.0 for the traditional 95% reference interval based on x +/- 2*s(total) (s(total) = total biological variation), and also for a decision limit (cut-off point) x +/- 3*s(total). The numbers are, of course, different, but the effects of the index of individuality are the same, independent of the chosen cut-off point. This concept is related to the clinical classification (diagnosis, prognosis, screening) and the difference from different principles of monitoring is discussed. Further, five examples are evaluated and aspects of index of individuality in relation to false-positive results are discussed.     

Analytical goals for the estimation of non-Gaussian reference intervals

Goals for analytical quality are postulated for the situation in which a number of laboratories will share common reference intervals for some quantities for which the population is homogeneous. These goals are evaluated on the assumption that the acceptable combined analytical bias and imprecision should not allow the percentage of the population outside either upper or lower reference limits to exceed the interval 1.3 to 4.4%. The goals are evaluated for log-Gaussian biological distributions, and for distributions which can be transformed into log-Gaussian by addition of a constant value to all elements. Further analytical goals for non-parametric distributions are discussed. For log-Gaussian distributions the maximum allowable bias and imprecision are dependent on the ratio of the upper and lower reference limits. When this ratio is c. 1.25, the bias alone must be less than 1.5% or the coefficient of variation alone less than 3.5%. For a ratio of 10, these values may be as high as 15% and 34%, respectively.     

Models for combining random and systematic errors. assumptions and consequences for different models.

A series of models for handling and combining systematic and random variations/errors are investigated in order to characterize the different models according to their purpose, their application, and discuss their flaws with regard to their assumptions. The following models are considered 1. linear model, where the random and systematic elements are combined according to a linear concept (TE = absolute value(bias) + z x sigma), where TE is total error, bias is the systematic error component, sigma is the random error component (standard deviation or coefficient of variation) and z is the probability factor; 2. squared model with two sub-models of which one is the classical statistical variance model and the other is the GUM (Guide to Uncertainty in Measurements) model for estimating uncertainty of a measurement; 3. combined model developed for the estimation of analytical quality specifications according to the clinical consequences (clinical outcome) of errors. The consequences of these models are investigated by calculation of the functions of transformation of bias into imprecision according to the assumptions and model calculations. As expected, the functions turn out to be rather different with considerable consequences for these types of transformations. It is concluded that there are at least three models for combining systematic and random variation/errors, each created for its own specific purpose, with its own assumptions and resulting in considerably different results. These models should be used according to their purposes.     

AIDS, nursing and occupational risk: an ethical analysis

This paper examines the notion of risk assessment in relation to nursing patients with HIV infection or AIDS. Risk assessment consists in examining three features of a given hazard; its magnitude, its probability, and its acceptability. In relation to the last of these, an 'acceptability threshold' can be identified. If contact with HIV-infected patients is perceived to pass this threshold, nurses may seek to abrogate the usual 'duty to treat'. Rather than endeavour to pinpoint this threshold, the paper explores some of the factors which may either raise or lower it, and their ethical implications. In particular, the consequences of setting a high threshold, and thereby limiting or avoiding contact with patients infected with HIV, are examined. In this debate, the burden of proof is seen as resting firmly on nurses themselves, not on the patients at risk of being deprived of care.     

Improved allocation of costs through analysis of variation in data: planning of laboratory studies.

BACKGROUND: When developing a new laboratory test for study of human diseases, it is important to identify and control internal and external sources of variation that affect test results. It is also imperative that the precision of the test not only meets pre-established requirements and not exceed allowable total error, but also that these objectives are reached without undue expenditure of either time or financial resources. METHODS: This study applies statistical principles in designing a cost-effective experimental approach for determining the analytical precision of a new test. This approach applies the statistical concept of variance components to the problem of balancing a pre-established level of analytical precision against expenses incurred in achieving this precision. RESULTS: We demonstrated (1) estimation of variance components, (2) use of these estimates for improving allocation of costs within the experiment, and (3) use of these estimates for determining the optimal number of replicate measurements. CONCLUSIONS: Although elimination of all sources of variation that can affect laboratory test results is unlikely, the application of analysis of variance (ANOVA) statistical techniques can lead to a cost-effective allocation of resources for estimating the precision of a laboratory test.     

Use and interpretation of HbA1c testing in general practice. Implications for quality of care

We evaluate the interpretation and use of HbA1c results in general practice in relation to diabetes follow-up and case-finding. As part of an external quality assurance scheme for laboratory analyses, two case histories were mailed to all Norwegian GPs with equipment for HbA1c analysis in their office laboratory (n=566) and to a random sample of GPs without such instruments (n=419). Patient A represented a monitoring situation and the GPs were asked to state changes in HbA1c signifying any improvement or deterioration of metabolic control. The initial HbA1c value stemmed from analysis of quality control material in the instrument group and was a preset value for the other group. In patient B, we focused on the use of HbA1c and other laboratory tests in diabetes case-finding. In the monitoring situation, 22% of GPs in the instrument group misclassified changes in HbA1c values, since these were less than the analytical uncertainty. Further, when interpreting HbA1c results, 64-93% of GPs (i) assumed that analytical quality was better than it really was, (ii) did not appreciate biological variation of HbA1c, or (iii) acted on small differences to be on the safe side. In case-finding, HbA1c was deemed important by 29% of GPs; doctors choosing not to perform a glucose tolerance test relied more on HbA1c. GPs have to be aware of analytical quality and biological variation when interpreting HbA1c results. The present limitations of HbA1c in the diagnosis of diabetes are not properly understood.     

Analytical quality specifications for common reference intervals.

Interpretation of laboratory test results requires comparison to some type of reference value or reference interval. These comparisons can be cross-sectional (population-based reference interval and cut-off values) or longitudinal (reference change value). Quality specifications for cross-sectional comparison have been established by determining the influence of analytical bias and imprecision on the percentage of the healthy population falling outside the reference limits, when sharing population-based reference intervals in a Gaussian distribution of results. Quality specifications for longitudinal comparisons are equally important and are often overlooked, since less work has been done in this area. Some criteria suggest that a difference between consecutive results designates a true change in a patient health status when the difference is higher than the within-subject biological variation plus the within-laboratory analytical variation. In this chapter we discuss the clinical considerations and laboratory-related factors that must be considered when quality specifications are applied to sharing reference comparisons. Real life experience shows that different analytical methods can produce comparable results when common quality goals are established, and quality can be achieved through a willingness to work together. Within the existing organization, the current specifications for analytical quality and a dedication to quality health care makes it possible to achieve transferability between laboratories within a geographic area.     

Clinical assessment of haemoglobin values by general practitioners related to analytical and biological variation

Twelve case stories were mailed to 273 general practitioners to study the clinical assessment of haemoglobin values. For each case the general practitioner was asked to fill in the haemoglobin value representing the minimal change from a given value considered necessary to take action. haemoglobin change corresponding to the median haemoglobin value stated was transformed to a so-called 'medically useful coefficient of variation' in order to relate clinical demands to analytical and biological variation. The average medically useful coefficient was calculated to 3.9% (range 2.3-7.8%). We found that general practitioners assess haemoglobin values rather uniformly, although their judgement varies substantially with the clinical situation; they are not fully aware of the consequences of analytical and biological variation, and the advantage of knowing a previous haemoglobin value is not recognized. The analytical imprecision of haemometers based on clinical demands should be 2.8%, and the analytical quality should be the same in primary and secondary care.     

Use and interpretation of HbA1c testing in general practice. Implications for quality of care

We evaluate the interpretation and use of HbA_1c results in general practice in relation to diabetes follow-up and case-finding. As part of an external quality assurance scheme for laboratory analyses, two case histories were mailed to all Norwegian GPs with equipment for HbA_1c analysis in their office laboratory (n=566) and to a random sample of GPs without such instruments (n=419). Patient A represented a monitoring situation and the GPs were asked to state changes in HbA_1c signifying any improvement or deterioration of metabolic control. The initial HbA_1c value stemmed from analysis of quality control material in the instrument group and was a preset value for the other group. In patient B, we focused on the use of HbA_1c and other laboratory tests in diabetes case-finding. In the monitoring situation, 22% of GPs in the instrument group misclassified changes in HbA_1c values, since these were less than the analytical uncertainty. Further, when interpreting HbA_1c results, 64-93% of GPs (i) assumed that analytical quality was better than it really was, (ii) did not appreciate biological variation of HbA_1c, or (iii) acted on small differences to be on the safe side. In case-finding, HbA_1c was deemed important by 29% of GPs; doctors choosing not to perform a glucose tolerance test relied more on HbA_1c. GPs have to be aware of analytical quality and biological variation when interpreting HbA_1c results. The present limitations of HbA_1c in the diagnosis of diabetes are not properly understood.     

The relationship among mathematics anxiety, beliefs about mathematics, mathematics self-efficacy, and mathematics performance in associate degree nursing students

This research explored nursing students' mathematics anxiety, beliefs about mathematics, and mathematics self-efficacy in relation to performance on a medication mathematics test. Results revealed that the participants experienced some mathematics anxiety and had positive beliefs about mathematics and mathematics self-efficacy. Qualitative responses indicated that participants worried about the consequences of failing the medication mathematics test and that practice helped reduce this anxiety. In addition, participants acknowledged the importance of correct dosage calculations for nursing practice. Implications for nursing education are discussed.     

Seasonal and biological variation of urinary epinephrine, norepinephrine, and cortisol in healthy women.

BACKGROUND: There is a significant circadian and seasonal periodicity in various endocrine functions. The present study describes the within-day and seasonal fluctuation for urinary catecholamines and cortisol and estimates the within- (CV(i)) and between-subject (CV(g)) coefficients of variation for healthy women undertaking their routine work. In addition, index of individuality (I(i)) and power calculations were derived. METHODS: Eleven healthy females undertaking their routine life-style at work participated in the study. Each subject collected six samples during 24 h 15 days over a year, giving a total number of 990 samples. Using a random effect analysis of variance, we estimated CV(g) and total within-subject variation (CV(ti)), i.e. combined within-subject and analytical variation, from logarithmically transformed data. Analytical variation was subtracted from CV(ti) to give CV(i). CV(i) was estimated from samples collected monthly during 1 year (CV(iy)), weekly during 1 month (CV(im)), and six to eight times/day (CV(id)). RESULTS: A seasonal variation was demonstrated for excretion of epinephrine, norepinephrine, and cortisol standardized with creatinine. Concentrations of urinary epinephrine were higher during June and July compared to the rest of the year, whereas concentrations of urinary cortisol were higher during December and January compared to the rest of the year. Excretion of norepinephrine was lower during working hours and higher during hours off work for June and July compared to the rest of the year. There was a high within- and between-subject variation, which could not be explained by menstrual cycle, behavioral, emotional, or cognitive stress reactions. CONCLUSIONS: Despite high biological variation a reasonably low sample size, e.g. 10-50 individuals, is adequate for practical applicability, i.e. studying differences above 150%. The present study recommends to include the sampling time in the statistical evaluation of data and to be aware of the changes in diurnal variations over seasons. When single measurements are to be evaluated, reference intervals are recommended.     

Screening for haemochromatosis: influence of analytical imprecision, diagnostic limit and prevalence on test validity

Serum ferritin (S-ferritin) and the saturation of transferrin iron-binding capacity (TIBC-sat) were evaluated as screening procedures for idiopathic haemochromatosis in a non-diseased population. Special attention was paid to the influence of the analytical quality, but the effects of prevalence and discrimination limits were also considered. Changes in the analytical quality for S-ferritin and TIBC-sat used as single tests highly influenced the number of false test-positives. Increasing the coefficient of variation from 0.0 to 0.2 resulted in nearly a doubling of the number of false test-positives to be further investigated. Using even a high, yet achievable analytic quality with a low coefficient of variation of 0.056 and 0.059 for S-ferritin and TIBC-sat respectively, screening procedures had unacceptably high fractions of false test-positives and false test-negatives associated with any discrimination limit. If the prevalence of haemochromatosis is 0.003, the predictive value of a positive test result did not exceed 0.05, accepting a fraction of false test-negatives of 0.025. This was found to be too low for a screening test to be used in the general population. The combined use of S-ferritin and TIBC-sat resulted in higher performance with a sensitivity of 0.90, a specificity of 0.99 and a predictive value of a positive test result of 0.29 if a fraction of misclassification of 0.01 is accepted for each.     

Biological variation of asymmetric dimethylarginine and related arginine metabolites and analytical performance goals for their measurement in human plasma

BACKGROUND: Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide synthase which is believed to be a cause of endothelial dysfunction and has been shown to predict the occurrence of acute coronary events. Data regarding the biological variation of arginine and its methylated derivatives are conspicuously absent from the literature. Such data are important in setting analytical quality specifications, assessing the utility of population reference intervals and assessing the significance of changes in serial results from an individual. MATERIALS AND METHODS: Arginine, homoarginine, ADMA and symmetric dimethylarginine (SDMA) are measured in plasma by high performance liquid chromatography. Twelve healthy volunteers underwent weekly blood sampling for 20 weeks in order to determine the intra- and inter-individual biological variation of these analytes, from which analytical quality specifications, indices of individuality (II) and reference change values (RCV) are derived. Plasma samples from 100 healthy individuals were obtained in order to determine population reference intervals. RESULTS: ADMA and symmetric dimethylarginine (SDMA) exhibit low intra-individual biological variation of 7.4% and 5.8%, respectively, imposing desirable imprecision goals (CV(A)) of < or = 3.7% and 2.9% for these analytes. The described methodology achieves these goals, with analytical CVs of < 3.5% for all analytes. Goals for bias and total error were 3.1-10.1% and 7.2-16.0%, respectively. Reference intervals for ADMA and SDMA were 0.29-0.63 micromol L(-1) and 0.24-0.55 micromol L(-1), but have IIs < 1. RCVs were at least 20% for all analytes studied. CONCLUSIONS: Dimethylarginine concentrations are tightly controlled in health, with the result that imprecision goals for laboratory methods require to be low. Relatively large differences are required between serial results to denote a significant change. Population reference intervals for dimethylarginines are likely to be of limited value in detecting 'abnormality' in an individual from a single result.     

Assessing physicians' compliance with guidelines for Papanicolaou testing.

In this study, population-based data were used to examine the appropriateness of Papanicolaou (Pap) testing from the perspective of the women being tested and their physicians. The approach used is unique in its assessment of overtesting and undertesting in the primary care setting. From the data base of the province of Manitoba's universal health insurance plan, 4-year health histories (1981 to 1984) were constructed for each woman from a random sample of the population of women who, in 1982, were between the ages of 25 to 64 years (n = 22,287). At the last visit to a general practitioner, gynecologist, or general surgeon in 1984 (termed the current visit), the authors determined whether a Pap test was given for each woman. Using decision rules from a Canadian task force report on cervical screening and previous health history, the authors evaluated the appropriateness of screening by determining whether a Pap test was given and was needed, or whether a women who had not received a Pap test required one. Overall, 55.7% of women were tested appropriately. Of the 5352 women who received a Pap test at the current visit, 62.8% were overtested. Of the 16,935 women not tested at the current visit, 38.5% required screening (i.e. were undertested). Characteristics of a physician's practice that were significantly related to compliance with the guidelines included having a high proportion of patients visiting for obstetric or gynecologic reasons. Variables that were associated with negative compliance were 1) being a gynecologist; and 2) having a high proportion of patients who lived in inner city or rural areas. Because physicians are paid a fee for every Pap smear taken and the guidelines were well disseminated, these results should be reasonably representative of fee-for-service practice in North America, where preventive care is not subject to user charges. This study supports previous findings that a passive approach to dissemination of guidelines is insufficient to effect practice.     

Real-time spatial compound imaging improves reproducibility in the evaluation of atherosclerotic carotid plaques.

Compound imaging has the ability of reducing speckle and clutter artifacts demonstrated in in vitro studies compared to conventional, single-angle imaging. We investigated intra- and interobserver agreement of 38 outlines of carotid artery plaque images acquired by these techniques, by measuring the overlapping area after repeated outlines. In general, both techniques showed good agreement. When considering the images with poorest overlap, compound imaging had a significant advantage over conventional imaging regarding both intra- and interobserver agreement. The interobserver variation for the overlapping area after two outlines was 20% for conventional technique and 10% for compound. The interobserver variation of the gray scale median value (GSM) for conventional technique ranged from -32 to +20 and from -6 to +6 for compound. Likewise, the coefficient of repeatability for the GSM value was 13 for conventional imaging and three for compound imaging, and interobserver variation for the GSM value for the overlapping area was 34% and 9% for conventional and compound technique. In conclusion, compound imaging improves intra- and interobserver agreement and reduces interobserver variation in the GSM value in a clinical setting.     

The relation between atypical migraine and multiphasic oral contraceptives.

Data in references referring to the relation between atypical or menstrual migraine and the use of oral contraception have been reviewed. The authors own observations are discussed on this subject. Motilium and Voltaren have been successfully used for controlling vascular headache developing as a side-effect of contraceptive tablets. In case of migraine associated with dysmenorrhoea and/or premenstrual tension the management with triphasic hormone proved to be of therapeutic value. The authors call attention to the concurrent high incidence of these latter symptoms and suggest the further analysis of this problem. They collect evidence regarding that in the major part of cases atypical migraine is hormone dependent.     

Detection of gonococcal infection : pros and cons of a rapid test.

WHO estimates that 62 million cases of gonorrhea occur annually worldwide. Untreated infection can cause serious long-term complications, especially in women. In addition, Neisseria gonorrheae infection can facilitate HIV transmission, and babies born to infected mothers are at risk of ocular infection, which can lead to blindness.Where diagnostic facilities are lacking, gonorrhea can be treated syndromically. However, this inevitably leads to over-treatment, especially in women in whom the syndrome of vaginal discharge may be due not to N. gonorrheae infection but to several other more prevalent conditions. Over-treatment is a major concern because of widespread N. gonorrheae antibiotic resistance. Moreover, a high proportion of gonorrhea cases are asymptomatic and so do not present for syndromic management. Such cases will only be detected by screening tests.The gold standard test for the detection of N. gonorrheae is culture, which has high sensitivity and specificity. However, it requires well trained staff and its performance is affected by specimen transport conditions. Other options include microscopy and tests that detect gonococcal antigen or nucleic acid. Nucleic acid amplification tests (NAATs) have higher sensitivity and can be used on non-invasive samples (urine). However, they can cross-react with other Neisseria species and are expensive, requiring highly trained staff and sophisticated equipment.In settings where patients are asked to return for laboratory results, some infected patients never receive treatment as they fail to return for their test results. This reduction in treatment, and the possible onward transmission of N. gonorrheae during any delay in treatment, means that a rapid test of lower sensitivity may be more effective if it results in patients being treated at the initial visit. Indeed, even with the low sensitivity of currently available rapid tests (50-70%), modeling shows that they can outperform gold standard tests in populations with high sexual activity and/or low return rates. Unfortunately, however, most of the rapid tests currently available are immunoassays that are quite expensive and involve many steps, which limit their current usefulness.In summary, the pros and cons of using a rapid test are dependent on the setting. Culture or NAATs remain the best choice in an ideal setting. However, in settings where laboratory facilities are not available, or in high-risk populations where return rates are low, rapid tests may be the most effective way of diagnosing gonorrhea. Their optimal use in these settings requires the development of simpler and cheaper rapid tests.     

The effect of temperature variation on the accuracy of results in a continuous flow analysis system

The effects of temperature variation within a continuous flow system during an analytical run are discussed. Attention is drawn to the high incidence of increasing sensitivity and the effect of this on the method used for applying drift correction to the results. The increases in sensitivity have been found to be due not only to alterations in the dialysis temperature resultant upon altering ambient temperatures but mainly to heat gain within the colorimeter. Solutions to both these temperature problems are proposed, and these lead to the maintenance of constant sensitivity during test runs.     

Diagnosis and management of environmental thoracic emergencies

Physiologic sequelae from increasing ambient pressure in underwater activities, decreasing ambient pressure while at altitude, or the consequences of drowning present a unique set of challenges to emergency physicians. In addition, several environmental toxins cause significant respiratory morbidity, whether they be pulmonary irritants, simple asphyxiants, or systemic toxins. It is important for emergency physicians to understand the pathophysiology of these illnesses as well as to apply this knowledge to the clinical arena either in the prehospital setting or in the emergency department. Current treatment paradigms and controversies within these regimens are discussed.