Band 37, Heft 11-12

no abstract available

Clinical laboratory data is used to help classify patients into diagnostic disease categories so that appropriate therapy may be implemented and prognosis estimated. Unfortunately, the process of correctly classifying patients with respect to disease status is often difficult. Patients may have several concurrent disease processes and the clinical signs and symptoms of many diseases lack specificity. In addition, results of laboratory tests and other diagnostic procedures from healthy and diseased individuals often overlap. Finally, advances in computer technology and laboratory automation have resulted in an extraordinary increase in the amount of information produced by the clinical laboratory; information which must be correctly evaluated and acted upon so that appropriate treatment and additional testing, if necessary, can be implemented. Clinical informatics refers to a broad array of statistical methods used for the evaluation and management of diagnostic information necessary for appropriate patient care. Within the realm of clinical chemistry, clinical informatics may be used to indicate the acquisition, evaluation, representation and interpretation of clinical chemistry data. This review discusses some of the techniques that should be used for the evaluation of the diagnostic utility of clinical laboratory data. The major topics to be covered include probalistic approaches to data evaluation, and information theory. The latter topic will be discussed in some detail because it introduces important concepts useful in providing for cost-effective, quality patient care. In addition, an example illustrating how the informational value of diagnostic tests can be determined is shown.

This review deals with the six main clinical situations related to magnesium or one of its fractions, including ionized magnesium: renal disease, hypertension, preeclampsia, diabetes mellitus, cardiac disease, and the administration of therapeutic drugs. Issues addressed are the physiological role of magnesium, eventual changes in its levels, and how these best can be monitored. In renal disease mostly moderate hypermagnesemia is seen; measuring ionized magnesium offers minimal advantage. In hypertension magnesium might be lowered but its measurement does not seem relevant. In the prediction of severe pre-eclampsia, elevated ionized magnesium concentration may play a role, but no unequivocal picture emerges. Low magnesium in blood may be cause for, or consequence of, diabetes mellitus. No special fraction clearly indicates magnesium deficiency leading to insulin resistance. Cardiac diseases are related to diminished magnesium levels. During myocardial infarction, serum magnesium drops. Total magnesium concentration in cardiac cells can be predicted from levels in sublingual or skeletal muscle cells. Most therapeutic drugs (diuretics, chemotherapeutics, immunosuppressive agents, antibiotics) cause hypomagnesemia due to increased urinary loss. It is concluded that most of the clinical situations studied show hypomagnesemia due to renal loss, with exception of renal disease. Keeping in mind that only 1% of the total body magnesium pool is extracellular, no simple measurement of the real intracellular situation has emerged; measuring ionized magnesium in serum has little added value at present.

Over the past decades, the randomized controlled trial has entered an era of continuous improvement and has gradually become accepted as the most effective way of determining the relative efficacy and toxicity of new therapies because it controls for placebo and time effects. However, even sensitive and properly designed and executed trials do not always confirm hypotheses to be tested, and conclusions are not always confirmed by subsequent trials. Although the former may be due to wrong hypotheses, the latter is likely to be due to the presence of certain imperfections within the design and execution of the trial itself. In this opinion paper, while focusing on such imperfections, the author searched for methods for further improvement of controlled trials, particularly clinical trials. The examples used in this paper are obtained from literature search as well as recent studies performed by the Netherlands Working Group on Cardiovascular Research (WCN). Methods for improvement could include: 1. making every effort to avoid asymmetries in the treatment groups; 2. emphasis on statistical power rather than just null-hypothesis testing; 3. adjusting for asymmetries not only of patient characteristics but also of outcome variables; 4. accounting routinely for type III errors; 5. routinely weighing benefits of a new drug against risks.

A Search algorithm included in the Opus software of Bruker (Germany) was evaluated for analysis of urinary stones. Three reference libraries containing respectively 85 (single components), 1,059 (binary mixtures) and 4,565 (ternary mixture) digitized spectra were created and used to identify unknown spectra (n=320), applying the automatic procedure. Identification of the major component was correct in 83% of cases but the percentage of identification significantly decreased for the second and the third components. In cases of identification of the two first components, quantitative assessment was correct within tolerance limits ± 15%. The computer results are judged unsatisfactory with regard to pathology because computer-aided identification is not sufficiently sensitive and specific to differentiate species with similar spectral pattern, even for the identification of main component, and also to detect minor components. It can be of assistance to guide spectral analysis, but it cannot replace human identification.

We present the adaptation of an IFCC method for α-amylase using 2-chloro-4-nitro-phenyl-α-D-maltotrioside as substrate (1) suited for routine work at 37℃. In the assay, a constant proportion of substrate, i. e. 92%, is directly converted to 2-chloro-4-nitrophenol and maltotriose. The method is based on multi- and univariate optimization leading to following measurement conditions: substrate, 2.25 mmol/l; chloride, 310 mmol/l; calcium 5.0 mmol/l; 4-morpholinoethanesulphonic acid, 50 mmol/l; pH 6.28. The assay may be carried out manually or by mechanized procedures, with substrate or sample start, and it shows these analytical properties in measuring amylase activity of sera: no lag phase, detection limit 2.9 U/l, linear range ≤820 U/l (for 300 s) or ≤1450 U/l (for 120 s of measurement), and total manual imprecision 3.2% (CV) at 46 U/l. Bilirubin ≤630 μmol/l, haemoglobin ≤6 g/l, triacylglycerols ≤30 mmol/l, heparin ≤100 kU/l, and glucose ≤120 mmol/l do not interfere. For adults, we established a preliminary 0.95-reference interval of 30–90 U/l not dependent on sex or age. A close association with the IFCC method demonstrates the reliable transfer of its measurement conditions to a robust routine method with minimal changes.

Anticoagulant-induced aggregation of platelets leads to pseudothrombocytopenia. Blood cell counters generally trigger alarms to alert the user. We describe an insidious case of pseudothrombocytopenia, where the complete absence of Coulter counter alarms both in ethylenediaminetetraacetic acid blood and in citrate or acid citrate dextrose blood samples was compounded by the fact that the massive aggregates were exclusively found at the edges of the blood smear. Non-recognition of pseudothrombocytopenia can have serious diagnostic and therapeutic consequences. While the anti-aggregant mixture citrate-theophylline-adenosine-dipyridamole completely failed in preventing pseudothrombocytopenia, addition of iloprost to anticoagulants only partially prevented the aggregation. Only the prior addition of gentamicin to any anticoagulant used resulted in a complete prevention of pseudothrombocytopenia and enabled to count accurately the platelets.

We evaluated a new liquid homogeneous assay for the direct measurement of high density lipoprotein cholesterol (HDL-C Plus) in seven laboratories. The assay includes two reagents which can be readily used in most available clinical chemistry analyzers. The total CVs of the new method were below 4.6% and the bias in relation to the designated comparison method was below 3.9%. The total error ranged between 4 to 7%. HDL-C values determined by this method were in good agreement with those obtained by the old homogeneous assay using lyophilized reagents, and other homogeneous and precipitation assays (0.944 < r < 0.996). The assay was linear up to at least 3.89 mmol/l HDL-C. Hemoglobin did not interfere, whereas in icteric samples slight deviations were observed. Lipemia up to 11.3 to 22.6 mmol/l triglycerides did not interfere with this homogeneous HDL-C assay. In samples of patients with paraproteinemia, discrepant results were seen. This liquid homogeneous HDL-C assay was easy to handle and produced similar results in all laboratories participating in this study. This method will enable clinical laboratories to reliably measure HDL-C for risk assessment of coronary heart disease.

The early release of cardiac markers is influenced by a variety of factors, the most important influence being their intracellular compartmentation. In contrast to the release of cytosolic proteins, the release of structurally bound proteins requires both a leaky plasma membrane and a dissociation or degradation of the subcellular structure, which is a slower process. Another major impact is the susceptibility to the degradation by cytosolic proteases, such as the calpains. The lysosomes are stable within the first 3–4 hours after onset of ischemia, and, therefore, their enzymes are not involved in the early degradation of structurally bound proteins. Troponin I and troponin T are substrates of μ-calpain. Current experimental as well as clinical results suggest that the molecular mass seems to be of minor importance for the pattern of appearance of myocardial proteins in blood after myocardial infarction. However, within the family of molecules with a certain intracellular compartmentation, the molecular mass is an influence on the appearance in blood, because heavier molecules diffuse at a slower rate, and particularly smaller molecules, such as myoglobin, may enter the vascular system to an even larger extent directly via the microvascular endothelium. The higher the concentration gradient of a marker between the cardiomyocytes and the interstitial space, the faster a parameter will translocate from sarcoplasma to the interstitial space as soon as the plasma membrane permeability is increased. Another influence is local blood and lymphatic flow. Recent experimental studies showed that reperfusion causes a true acceleration of cellular protein leakage by an acute manifestation of plasmalemmal disruptions and not just an enhanced wash out. Marker protein time-courses after myocardial damage are also markedly influenced by their disappearance rate from blood. Most proteins appear to be catabolized in organs with a high metabolic rate, such as liver, pancreas, kidneys, and the reticuloendothelial system. Smaller molecules, such as myoglobin, also pass the glomerular membranes of the kidneys and are reabsorbed and subsequently metabolized in tubular epithelial cells.

This paper reviews the tissue specificity of cardiac troponin I (cTnI), cardiac troponin T (cTnT) and creatine kinase (CK) MB in human and animal heart and skeletal muscles. Studies reveal that CK-MB can be expressed up to 20% of total CK activity in human skeletal muscle; and therefore is not 100% specific for the heart. One cTnI isoform has been described and shown to be 100% specific for the heart. While one to four cTnT isoforms are expressed in diseased and regenerating human skeletal muscle, these isoforms are not the same as the cTnT isoforms expressed in the human heart and are not detected by the cTnT diagnostic assays used in clinical practice. Representative cases are described demonstrating the role of monitoring cardiac troponins in blood for differentiating false positive CK-MB increases due to skeletal muscle injury. Further, sufficient reactivity and tissue specificity of cTnI and cTnT assays are demonstrated for use as markers of myocardial injury in laboratory animals. Monitoring cTnI and cTnT concentrations in the circulation appears poised as the new standards for detection of myocardial injury.

Troponin I (cTnI), a sensitive and reliable marker of damaged cardiac tissue, is now widely used in clinics. But the existence of different cTnI assays with a wide variety of cut-off values and discrepancies between the results of measurements of one and the same sample by different assays is puzzling for clinicians. The most urgent issue at the moment is the development of the international standard, which can be used for the calibration of different assays, thus decreasing between assay biases. But another important item, which should be considered by manufacturers, is the standardisation of the epitopes of the antibodies used for the assay development. The importance of such standardisation originates from the complicated biochemical nature of cTnI. Here we briefly try to analyse the main factors that can influence antigen recognition by different antibodies and formulate principles of antibody selection for assay development.

The aim of this study was to determine whether, using an evidence-based approach, the results of the papers found in the literature are valid and sufficiently scientifically rigorous to be used to definitely address the problem of cardiac marker sensitivity in detection of acute myocardial infarction. In particular, the diagnostic sensitivities of myoglobin, creatine kinase (CK)-MB isoenzyme, determined as mass concentration, CK-MB isoforms, and of the two cardiac troponins, troponin I and troponin T, were reviewed using a priori formulated inclusion/exclusion criteria for judging the eligibility of studies to be included in the analysis. A clear final message derived from this systematic analysis is the unacceptably poor diagnostic sensitivity of all evaluated markers at patient admission, with substantial failure rate to rule out myocardial infarction at this time. Myoglobin is at present the most sensitive of the markers studied for excluding early AMI with an optimum timing of sampling at patient presentation and approximately 4 h later. However, this marker cannot be used by itself as a proportion of patients admitted to the hospital with a late infarction could be missed. The early rate of rise of CK-MB mass and troponin T is similar. Maximum sensitivity of these two parameters is achieved by the analysis of a second sample 6 to 12 h after admission. Additional larger studies are needed to address the question which troponin shows earlier release after myocardial damage, and to clarify the role of CK-MB isoforms as a possible early marker of myocardial infarction.

Patients with chest pain represent an inhomogeneous group with greatly varying severity of coronary artery disease and cardiac risk. The proper selection of different treatment strategies in these patients requires reliable risk assessment. Patients with definitive myocardial infarction: in patients with ST-segment elevation on ECG, a positive troponin T (cTnT) on admission identifies a group of patients having a threefold higher mortality rate than patients with a negative cTnT test. The differences in risk based on cTnT are found for patients treated with thrombolytic as well as mechanical recanalization therapy. These differences in mortality based on admission cTnT may be explained by more severe coronary artery disease, worse left ventricular function, and less efficient microvascular reperfusion in the cTnT-positive patients. Patients with rest angina: in patients with angina at rest, a positive cTnT value on admission identifies a subgroup having a threefold higher cardiac event rate than cTnT-negative patients. The cTnT-positive patients seem to benefit from treatment with low molecular weight heparin and fibrinogen receptor antagonists, while cTnT-negative patients do not. The differences in risk and response to therapy may be due to more severe coronary artery disease, more critical coronary artery stenoses, and a higher rate of intracoronary thrombus formation in the cTnT-positive versus negative patients. Low risk chest pain patients: in low risk chest pain patients, (i.e. no rest angina, no ECG-changes) cTnT-positive patients on admission have a twofold higher cardiac event rate than cTnT-negative patients. The proper treatment strategy for the low risk cTnT-positive patients remains to be determined. Troponin T versus troponin I: many of the findings on cTnT also relate to troponin I. However, there is a high interassay variability of troponin I assays, which has to be taken into consideration.

Testing for the diagnosis of acute myocardial infarction and other diseases included in the spectrum of the “acute coronary syndrome” is rapidly changing from the traditional enzymatic assays to mass measurement of more specific and sensitive markers (cardiac troponins, CK-MB and myoglobin). Several questions have arisen since the introduction of these new markers into the clinical setting: the choice of strategies for optimizing the utilization of biochemical assays combining different (early and specific) markers, a rationale for sampling specimens and the identification of clinically useful turnaround times. In particular, for achieving the last goal, attention has been directed toward near-patient testing for cardiac markers in addition to, or as a replacement for, traditional diagnostic methodologies. While qualitative methods for measuring cardiac markers at the bedside have some limitations which compromise their clinical usefulness, new quantitative devices offer a real alternative to decentralized testing. Regulatory and quality management issues related to near-patient testing, as well as the performance of recently introduced devices for a decentralized measurement of cardiac markers are reviewed.

In modern medicine the undeniable value and indispensability of scientific investigations are now universally recognized both for diagnostic purposes and monitoring of disease and in basic epidemiology. The direct treatment of patients is an undeniable task of doctors in medicine. Progress in laboratory science is largely the result of contributions by scientists with an adequate education and specialisation in the field, i.e. by clinical chemists. Clinical laboratory science has developed on a broad front throughout the European Community, resulting in significant differences in what constitutes a national clinical chemistry service in each state. Clinical chemistry is the medical discipline devoted to obtain, explore and employ chemical knowledge and chemical methods of investigation, in order to procure knowledge about normal and abnormal chemical processes in man. These processes are studied on a general level, in order to get insight into human health and disease, and on a patient-specific level for diagnostic or monitoring purposes. The delimitation of clinical chemistry varies from country to country, since there is no sharp boundary to haematology, immunology, molecular biology and microbiology. One of the main tasks of the clinical chemist is direction and supervision of a laboratory department in a hospital or health service (public or private), where his role involves bridging the gap between rapidly developing laboratory science and technology and the growing knowledge on characteristics of disease. He must possess fundamental biochemical knowledge and have the ability to use this knowledge most appropriately as applied to clinical requirements, i.e. diagnosis of disease and planning and monitoring of therapy. Apart from providing a competent laboratory service, the clinical chemist must be able to function as a consultant to his clinical colleagues and liaise with them in the interpretation of laboratory results. His advice and professional consultation have at least three aspects, i.e. choosing the most appropriate laboratory investigation in a certain case, ensuring that the analyses are performed in the best possible way and correctly reported and, finally, providing information and (most important) interpretation on the significance and consequences of the laboratory data obtained. As the results of laboratory investigations and the consultation of the clinical chemist have a direct and important influence on the treatment of the patient, it is to the benefit of the public that the profession of the clinical chemist is duly regulated.