Quantitative turbidimetric assay for determining myoglobin evaluated

Pitfalls in the diagnosis of hematuria

Detection of hemoglobin (Hb) and red blood cells in urine (hematuria) is characterized by a large number of pitfalls. Clinicians and laboratory specialists must be aware of these pitfalls since they often lead to medical overconsumption or incorrect diagnosis. Pre-analytical issues (use of vacuum tubes or urine tubes containing preservatives) can affect test results. In routine clinical laboratories, hematuria can be assayed using either chemical (test strips) or particle-counting techniques. In cases of doubtful results, Munchausen syndrome or adulteration of the urine specimen should be excluded. Pigmenturia (caused by the presence of dyes, urinary metabolites such as porphyrins and homogentisic acid, and certain drugs in the urine) can be easily confused with hematuria. The peroxidase activity (test strip) can be positively affected by the presence of non-Hb peroxidases (e.g. myoglobin, semen peroxidases, bacterial, and vegetable peroxidases). Urinary pH, haptoglobin concentration, and urine osmolality may affect specific peroxidase activity. The implementation of expert systems may be helpful in detecting preanalytical and analytical errors in the assessment of hematuria. Correcting for dilution using osmolality, density, or conductivity may be useful for heavily concentrated or diluted urine samples.

Determinants of Clara cell protein (CC16) concentration in serum: a reassessment with two different immunoassays

Clara cell protein (CC16) is a 16 kiloDalton protein secreted by Clara cells in the lining fluid of bronchiolar and bronchial epithelium. Recently, Nomori et al., using a nephelometric latex immunoassay, reported a strong correlation between serum CC16 (sCC16) and serum lipids as well as the body mass index (BMI) [Nomori H, Horio H, Takagi M Kobayashi Y, Hirabayashi Y. Clara cell protein correlation with hyperlipidemia. Chest 1996;110:680-4]. The same authors found higher values of sCC16 in males compared to females and did not detect any significant influence of tobacco smoking. Since these results are in disagreement with previous observations showing consistently a decrease of sCC16 in smokers and no influence of sex, we have reassessed in healthy subjects the determinants of sCC16 using two different assays: a particle counting-based latex immunoassay (LIA) using polyclonal antibodies and a fluorescence enzyme immunoassay (FEIA) using monoclonal antibodies. sCC16 was determined in a group of 52 female and 44 male healthy subjects (age 18 to 66 years), including 35 smokers and 61 nonsmokers. sCC16 measured by LIA and FEIA were well correlated (r = 0.92, n = 96, P < 0.0001) with values (geometric mean and range) of 13.3 (5.2-34.5) and 14.7 (4.1-53.1) microg/l, respectively. The determinants of sCC16 measured by both techniques were traced by stepwise regression analysis using as independent variables age, sex, smoking status, BMI or serum lipids (total cholesterol and triglycerides) and the glomerular filtration rate (GFR) estimated on the basis of serum creatinine or beta2-microglobulin. Only two significant determinants emerged: tobacco smoking which correlated negatively and the GFR which correlated positively with sCC16. No influence of serum lipids, BMI, age and sex on sCC16 was detected. We think that an analytical interference with serum lipids explains the results by Nomori et al. which are not confirmed here by two independent techniques and are inconsistent with the current understanding of the physiopathology of the Clara cell and its main secretory product, CC16.

Diagnostic strategies using myoglobin measurement in myocardial infarction

Myoglobin, a low molecular-weight heme protein (17800 D) present in both cardiac and skeletal muscle, is an old test with new perspectives. Advantages and disadvantages of myoglobin determination are well known. Myoglobin is the earliest known, commercially available, biochemical marker of acute myocardial infarction (AMI) and its rapid kinetics make it an early, good marker of reperfusion. However, since myoglobin is present in both skeletal and cardiac muscle, any damage to these muscle types results in its release into blood. Serum myoglobin levels are falsely elevated in conditions unrelated to AMI as skeletal muscle and neuromuscular disorders, renal failure, intramuscular injection, strenuous exercise, and after several toxins and drugs intake. New strategies for myoglobin measurement may resolve this limitation. These strategies include both the combined measurement of myoglobin and a skeletal specific marker (carbonic anhydrase III) or a cardiac specific marker (troponin I), as well as the myoglobin evaluation on serial samples. In particular, the diagnostic algorithm based on the combined measurement of myoglobin and troponin I, assuring a satisfactory analytical turnaround time, significantly improves the diagnostic efficiency of laboratory assessment of suspected AMI patients, allowing the successive monitoring of coronary reperfusion.

Clinical evaluation is better: myoglobin estimation used singly as a discriminant for early acute myocardial infarction does not well identify patients who will benefit from thrombolytic therapy

We have examined the use of serum myoglobin concentration in the management of cases of suspected acute myocardial infarction (AMI). In a series of 51 patients myoglobin, used as a discriminant, correctly identified 97% (28/29) of cases as AMI with one false positive. Initial clinical judgement based on history, examination and the electrocardiogram correctly identified 66% (19/29) of cases with one false positive. These patients were given streptokinase. However, in these further identified AMI patients, 78% (7/9) had small enzyme rises with non-Q wave infarction and/or non-ST elevation and therefore may not have benefited from thrombolytic therapy in contrast to the Q wave/raised ST segment infarcts with large enzyme rises identified by clinical means. Enthusiasm for myoglobin estimation, where used as a discriminant for AMI, as a direct pointer to thrombolysis in the early diagnosis of AMI should be tempered by this finding.

Estimation of myocardial infarct size from plasma myoglobin or fatty acid-binding protein. Influence of renal function

Myoglobin (M(r) 18,000) and fatty acid-binding protein (M(r) 15,000), are low molecular mass cytoplasmic proteins that are considered useful biochemical markers for early detection or exclusion of acute myocardial infarction, and also for early estimation of infarct size. As each of these proteins shows renal clearance, we studied the influence of renal function on the estimation of infarct size from their plasma concentration curves. For this, infarct size estimated from plasma myoglobin or fatty acid-binding protein release curves was compared with that estimated with the established infarct size markers hydroxybutyrate dehydrogenase and creatine kinase, which are not influenced by changes in renal function. The discordance between infarct size estimates was related to renal function. Creatine kinase (EC 2.7.3.2), hydroxybutyrate dehydrogenase (EC 1.1.1.27), myoglobin, fatty acid-binding protein and creatinine were assayed serially in plasma samples obtained frequently and for at least 72 hours after the start of thrombolytic therapy in 20 patients with acute myocardial infarction. Cumulative release of the different cardiac markers was calculated by using a two-compartment model for circulating proteins. Mean tissue contents of 156 U/g for hydroxybutyrate dehydrogenase, 2163 U/g for creatine kinase, 2.79 mg/g for myoglobin and 0.57 mg/g wet weight for fatty acid-binding protein, were used to express infarct size in gram-equivalents of healthy myocardium per litre of plasma (g-eq/l). Mean plasma creatinine was obtained by averaging the creatinine concentrations measured in all plasma samples taken during the first 24 hours after acute myocardial infarction. A relation was found between the mean plasma creatinine concentration during the first 24 hours after acute myocardial infarction and the discordance between infarct size estimated from cumulative hydroxybutyrate dehydrogenase release, compared to infarct size estimated from cumulative myoglobin or fatty acid-binding protein release. For patients with mean plasma creatinine concentrations within the reference interval for creatinine (group 1, n = 15) a good agreement was found between infarct size estimated from myoglobin or fatty acid-binding protein plasma curves and that estimated with either hydroxybutyrate dehydrogenase or creatine kinase. However, for patients with a mean creatinine concentration above the upper reference limit (group 2, n = 5), infarct size calculated from plasma myoglobin or fatty acid-binding protein release curves was markedly overestimated, especially for larger infarcts. Estimation of infarct size from serial plasma myoglobin or fatty acid-binding protein concentrations is possible in the first 24 hours after the onset of symptoms, but only in patients with normal renal function, as estimated from plasma creatinine concentrations.

Discrimination between myocardial and skeletal muscle injury by assessment of the plasma ratio of myoglobin over fatty acid-binding protein

BACKGROUND

Myoglobin and fatty acid-binding protein (FABP) each are useful as early biochemical markers of muscle injury. We studied whether the ratio of myoglobin over FABP in plasma can be used to distinguish myocardial from skeletal muscle injury.

METHODS AND RESULTS

Myoglobin and FABP were assayed immunochemically in tissue samples of human heart and skeletal muscle and in serial plasma samples from 22 patients with acute myocardial infarction (AMI), from 9 patients undergoing aortic surgery (causing injury of skeletal muscles), and from 10 patients undergoing cardiac surgery. In human heart tissue, the myoglobin/FABP ratio was 4.5 and in skeletal muscles varied from 21 to 73. After AMI, the plasma concentrations of both proteins were elevated between approximately 1 and 15 to 20 hours after the onset of symptoms. In this period, the myoglobin/FABP ratio was constant both in subgroups of patients receiving and those not receiving thrombolytics and amounted to 5.3 +/- 1.2 (SD). In serum from aortic surgery patients, both proteins were elevated between 6 and 24 hours after surgery; the myoglobin/FABP ratio was 45 +/- 22 (SD), which is significantly different from plasma values in AMI patients (P < .001). In patients with cardiac surgery, the ratio increased from 11.3 +/- 4.7 to 32.1 +/- 13.6 (SD) during 24 hours after surgery, indicating more rapid release of protein from injured myocardium than from skeletal muscles.

CONCLUSIONS

The ratio of the concentrations of myoglobin over FABP in plasma from patients with muscle injury reflects the ratio found in the affected tissue. Since this ratio is different between heart (4.5) and skeletal muscle (20 to 70), its assessment in plasma allows the discrimination between myocardial and skeletal muscle injury in humans.

Is rapid myoglobin measurement of diagnostic value in the emergency presentation of non-traumatic chest pain?

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A rapid assay for the quantification of myoglobin: evaluation and diagnostic relevance in the diagnosis of acute myocardial infarction

We evaluated a new, fast, quantitative, turbidimetric assay (TurbiTimeSystem, Behringwerke AG, Marburg, Germany) for the determination of myoglobin concentration in serum. Within-run imprecision (n = 10) was < 3.7% in controls ranging from 81.1 to 621.4 micrograms/l and between-day imprecision (n = 50) was < 6% in controls ranging from 69.5 to 623.4 micrograms/l. The assay is linear over the measuring range and interfering substances such as bilirubin, haemoglobin or haptoglobin do not interfere but triacylglycerol-rich samples are only measurable after brief ultracentrifugation. EDTA- or citrate-treated samples display depressed myoglobin concentration when compared with serum samples. The upper reference limit for apparently healthy individuals (n = 100, 50 female and 50 male) is 61.5 micrograms/l. Comparison with nephelometry revealed a good correlation (r = 0.982) between the two methods with the regression equation: turbidimetric assay = 5.53 + 1.02x nephelometric assay. Serial determination of myoglobin concentration and creatine kinase in 18 patients with proven acute myocardial infarction showed in general an equal diagnostic significance for both analytes. In the first 4 hours after onset of chest pain, the determination of myoglobin can have an advantage, since it is released into the blood stream at an earlier stage, but thereafter myoglobin can lead to false negative diagnosis. Therefore, determination of creatine kinase and its isoenzyme MB is still the diagnostic strategy of choice in the diagnosis of acute myocardial infarction.

Plasma myoglobin in the early diagnosis of acute myocardial infarction

Serum and plasma myoglobin and creatine kinase-MB catalytic activity were analysed in 157 patients admitted within 2 hours of the onset of chest pain (58 were retrospectively recognized as acute myocardial infarction). Serum and plasma values were highly correlated for both myoglobin and creatine kinase-MB. Plasma myoglobin appeared to be more sensitive than creatine kinase-MB for the early diagnosis of acute myocardial infarction; using a cut-off value of 100 micrograms/l, 90% of acute myocardial infarction cases were correctly recognized by plasma myoglobin 6 hours after the onset of chest pain, with a diagnostic specificity of 100% for non-acute myocardial infarction chest pain subjects. Plasma creatine kinase-MB showed a diagnostic sensitivity of 62% and a diagnostic specificity of 95% in the same group of patients. We suggest the inclusion of the plasma myoglobin immunonephelometric assay together with plasma creatine kinase-MB activity analysis in protocols for the early diagnosis of acute myocardial infarction.

The earliest diagnosis of acute myocardial infarction

Acute myocardial infarction results from the cessation of myocardial blood flow caused by thrombotic occlusion of a coronary artery. Rapid restoration of blood flow to the ischemic myocardium minimizes cardiac damage and improves early and long-term morbidity and mortality. Chest pain is the first symptom of myocardial infarction, but in some patients with silent ischemia, the disease can be diagnosed only in retrospect. In symptomatic patients, myocardial infarction should be accurately and promptly diagnosed so that reperfusion therapy can begin immediately. Electrocardiography is the simplest diagnostic modality. Although regional ST-segment elevation is specific, it is not sensitive. In contrast, new computerized algorithms for electrocardiographic analysis and serial monitoring increase sensitivity without decreasing specificity. In the emergency room, echocardiography is used to diagnose patients with no prior history of coronary artery disease whose electrocardiograms proved nondiagnostic. Time-consuming perfusion nuclear studies are inferior to echocardiography but may nevertheless enable physicians to diagnose myocardial infarction in the emergency room. Although the presence of excess creatine kinase is a sign of myocardial necrosis, its increase is delayed for a few hours after coronary occlusion. Doctors can diagnose myocardial infarction as early as two hours after coronary occlusion with the help of simpler automatic assays of MB-creatine kinase mass that use monoclonal antibodies. Other investigational markers of myocardial necrosis include myoglobin and troponin. Elevation of a circulating protein marker also signifies established necrosis, but physicians hope to achieve reperfusion through therapy before irreversible damage occurs.

Noninvasive assessment of reperfusion and reocclusion after thrombolysis in acute myocardial infarction

The clinical significance of ST-segment changes and of the time course of appearance in serum of different cardiac proteins has been reviewed for the diagnosis of coronary reperfusion and reocclusion after thrombolysis. In particular, the value of serial 12-lead electrocardiographic (ECG) studies, of Holter monitoring, and of continuous multilead computer-assisted ECG monitoring is compared. Regarding the serum proteins, the clinical significance of reperfusion indices described so far for serum creatine kinase (CK), its isoenzyme serum creatinine kinase MB, the CK isoforms, and myoglobin is reviewed. Emphasis is placed on (1) the calculation method used for deriving the reperfusion indices; (2) the sensitivity and the specificity of the reperfusion indices; (3) the minimum turn-around time needed to produce the reperfusion indices (depending on the practicability of the analytical and calculation methods and their applicability in an emergency laboratory); (4) the ability of the indices to produce reliable estimates of reperfusion efficacy of the thrombolytic agents under study; and (5) the ability of the marker proteins to detect reinfarction as well as the suitability of the markers to detect real-time necrosis.

Early diagnosis of acute myocardial infarction: CK-MB and myoglobin compared

We have measured changes in plasma concentration of creatine kinase MB (CK-MB) and myoglobin in 50 patients admitted to the Coronary Care Unit with chest pain of presumed cardiac origin. Eight serial blood samples were obtained in the 6 h period following admission and both CK-MB and myoglobin concentrations were measured. We compared the performance of single values of both tests. Myoglobin concentration, in the coronary care population studied, proved to be as specific as CK-MB concentration (92.6% in both cases) but with sensitivity of 100% being achieved 1.5 h post admission rather than 4 h post admission in the case of CK-MB. On this evidence, measurement of plasma myoglobin could prove useful in the rapid diagnosis of myocardial infarction with consequent effects on optimal Coronary Care utilisation and selection of patients for thrombolytic therapy.

Acute myocardial infarction size and myoglobin release into serum

The kinetics of myoglobin release after acute myocardial infarction were studied. Various algorithms for calculation of infarct size, based on immunonephelometric determination of myoglobin and cumulative myoglobin release into the circulation were compared. The cumulative myoglobin release and maximal serum myoglobin concentration were compared with various measures of infarct size: cumulative release of creatine kinase, electrocardiographic changes, and left ventricular ejection fraction. After acute myocardial infarction, time to peak for myoglobin in serum was correlated with time to peak for creatine kinase (r = 0.645). On average, the myoglobin concentration peaked 8.8 h earlier than creatine kinase activity. The rate of elimination of myoglobin showed a large variation (0.041-0.628 h-1) and was not correlated with the elimination rate of creatine kinase. The elimination rate of myoglobin after acute myocardial infarction was shown to depend on the patient's age and infarct size. The elimination constant of myoglobin is preferably estimated on an individual basis in large and complicated infarctions. Cumulative myoglobin release correlated with algorithms based on the cumulative release of creatine kinase (r = 0.622) and its isoenzyme MB (r = 0.660), and to a lesser extent with the residual left ventricular ejection fraction (r = 0.513) and the sum of ST-segment deviations on electrocardiography (r = 0.469). Maximal myoglobin values in serum correlated moderately with the calculated infarct size (r = 0.488; based on creatine kinase-MB) and electrocardiographic changes (r = 0.554). In combination with fast immunological methods for myoglobin determination, myoglobin peak height offers the advantage of providing reliable results within 12 h after onset of symptoms.(ABSTRACT TRUNCATED AT 250 WORDS)