Direct measurement of creatine kinase-MB activity in serum after extraction with a monoclonal antibody specific to the MB isoenzyme

Fusion of splenocytes from A/J mice immunized by creatine kinase (EC 2.7.3.2)-MB isoenzyme (CK-MB) with SP2/0-Ag14 myeloma cell line generated hybridomas producing monoclonal antibodies specific to CK-MB. One of these monoclonal antibodies ("Conan-MB") was used to develop a direct assay for CK-MB activity. In the assay, serum is incubated for 30 min at room temperature with "Conan-MB" immobilized on latex beads. The beads are then washed, and CK-MB activity bound to the antibody is measured after incubation with CK enzyme reagent for 30 min at 37 degrees C. Results with the assay correlated well (r = 0.997) with those for CK-MB concentration as measured by a two-site immunoassay. Neither CK-MM, CK-BB, mitochondrial CK, nor a hemolysate of erythrocytes interfered. Use of this unique monoclonal antibody allows rapid, precise, and direct determination of CK-MB activity.     

Immunochemiluminometric assay of creatine kinase MB with a monoclonal antibody to the MB isoenzyme

Previous two-site immunometric assays for creatine kinase (CK; EC 2.7.3.2) MB isoenzyme have been based on formation of a "sandwich" complex involving CK-MB and antibodies that recognize the CK-MM and the CK-BB isoenzymes. Single-incubation model assays of CK-MB with these antibodies were susceptible to interferences by CK-MM and CK-BB. We produced two anti-CK-MB monoclonal antibodies and studied their suitability for two-site assays. Both antibodies were compatible with anti-CK-MM and anti-CK-BB, but not with each other. Using anti-CK-MB as the tracer antibody eliminated the interference by both CK-MM and CK-BB. Labeling anti-CK-MB with acridinium ester and immobilizing anti-CK-BB on paramagnetic particles, we developed a rapid and highly sensitive chemiluminescent/magnetic separation CK-MB assay. As little as 1 microgram of CK-MB per liter was detectable after 10- or 30-min incubation at room temperature, and the standard curve was linear up to 400 micrograms/L. Results for serum samples by the new assay correlated well (r = 0.94) with those by Corning electrophoretic and the Hybritech Tandem-E immunoenzymometric CK-MB methods. Sera containing macro CK-1 or high concentrations of CK-MM and CK-BB did not interfere. The combined advantages of a more-specific antibody, paramagnetic solid phase, and chemiluminescent label endow this two-site CK-MB assay with performance characteristics and ease of use superior to those of previous assays.     

Immunochemical extraction and automated measurement of plasma creatine kinase MB isoenzyme and creatine kinase MB2 isoform

Measurement of creatine kinase MB (CK-MB) and its isoforms CK-MB₂ and CK-MB₁ are now applied in the diagnosis of acute myocardial infarction (AMI). The most common approach for analysis includes RIA, IRMA, and electrophoresis, all of which may be time-consuming. This study examines determination of CK-MB and CK-MB₂ by a rapid immunochemical extraction method followed by an automated measurement for both analytes. The automated method was sensitive to 2 U/L, linear to 180 U/L, and gave excellent interassay precision (<10% CV). Interference studies indicated that bilirubin, hemolysis, and lipemia caused analytical problems as did the presence of high activities of other CK isoenzymes, notably CK-MM and CK-BB, requiring dilution of samples prior to analysis. Application of immunochemical extraction gave a reference interval of CK-MB (0–2.5 U/L) and CK-MB₂ (0.1–1.4 U/L) for blood donors (20–60 years), peak levels for ruled-out AMI patients of CK-MB (0.5–7.3 U/L) and CK-MB₂ (0.3–4.9), peak levels for ruled-in AMI patients of CK-MB (80–174 U/L) and CK-MB₂ (80–155 U/L). Coronary artery bypass patients (n = 24) and all trauma patients (n = 14) also demonstrated elevations in CK-MB and CK-MB₂, whereas only five of the trauma patients demonstrated increased CK-MB by IRMA. In patients (n = 7) having increased total CK and normal CK-MB by IRMA, the extraction assay for CK-MB and CK-MB₂ yielded increased values in all patients. This new approach to CK-MB and CK-MB₂ analysis can be performed within 30 minutes of sample receipt. J. Clin. Lab. Anal. 11:163–168, 1997. © 1997 Wiley-Liss. Inc.     

At what level of serum total creatine kinase activity can measurement of serum creatine kinase MB isoenzyme activity be omitted in suspected myocardial infarction?

The purpose of this study was to establish a discriminatory limit for serum total creatine kinase activity (CK activity) below which CK isoenzyme fractionation is unnecessary. We looked at 2610 serum samples from 1077 consecutive patients with suspected acute myocardial infraction (AMI). The CK activity was determined according to the Scandinavian recommended method. Isoenzymes of CK were separated by agarose gel electrophoresis, followed by fluorometric scanning. When the threshold for CK activity was 150 U/l, none of the samples had a creatine kinase MB isoenzyme activity (CK-MB activity) equal to or higher than 30 U/l (the diagnostic level), which has been found to differentiate between patients with AMI and those without AMI. Only 14 patients (1.3% of all patients investigated) had CK-MB activity peaks between 10 U/l (detection limit) and 30 U/l. Of these, AMI was only diagnosed in one. We recommend that CK-MB activity should be measured only when CK activity is higher than 150 U/l. This would make about 50% of all CK-MB measurements unnecessary.     

Unexplained increase in serum creatine kinase isoenzyme MB activity in a lung cancer patient

In this case of mixed small cell--large cell cancer of the lung in an elderly woman, creatine kinase (EC 2.7.3.2) isoenzymes were assayed serially because of chest pain. The proportions of serum CK-BB and CK-MB isoenzyme activities were persistently above normal (CK-MB 10-18%, normal less than 5%). Electrocardiograms revealed no signs of ischemia or infarction. At autopsy no gross or microscopic infarction or inflammation of the heart was seen. There was also no infarction of smooth or skeletal muscle. The tumor was the probable source of most of the circulating CK-MB isoenzyme. Future cases may pose a similar diagnostic dilemma: differentiating creatine kinase that is present as a result of myocardial infarction from tumor-related CK-MB. Whether or not CK-MB assay could be useful in detecting tumors remains to be investigated.     

Increased activity of creatine kinase isoenzyme MB in a theophylline-intoxicated patient

A 78-year-old woman had increased activities of creatine kinase (CK; EC 2.7.3.2) and CK-MB isoenzyme in her serum, associated with severe theophylline intoxication. The time course for CK-MB activity was similar to that from an acute myocardial infarction. Clinical findings, however, including electrocardiograms, did not support the diagnosis of myocardial infarction. We suggest caution in interpreting CK-MB results in severe theophylline intoxication.     

Activity concentration and mass concentration (monoclonal antibody immunoenzymometric method) compared for creatine kinase MB isoenzyme in serum

Results of the "Tandem-E CKMB" immunoenzymometric procedure (y) for creatine kinase (CK; EC 2.9.3.2) were compared with electrophoresis (x) for 160 serum samples from patients suspected of having sustained myocardial infarctions. The results correlated well: y, microgram/L (Tandem assay) = 1.3x-6.3 U/L(electrophoresis) (r = 0.95). CK-MB mass measurement was more stable than enzyme activity after storage and appeared to be more sensitive. Sera from 86 other people, which had no detectable CK-MB upon electrophoresis, gave a mean CK-MB value of 1.1 microgram/L (SD 1.3, range 0-8) with the Tandem assay. To determine whether these low values represented actual isoenzyme, we tested for possible interference by heterophile antibodies in the patients' sera by preincubating the samples with mouse serum before the Tandem assay. The mouse serum did not interfere with the assay of sera that had substantial quantities of CK-MB by electrophoresis. However, in five of six samples that were negative by electrophoresis, the CK-MB values were substantially smaller, indicating that the values measured were false-positives caused by the presence of heterophile antibodies directed against mouse proteins, an interference that could be eliminated by pretreatment with mouse serum.     

Mass concentration and activity concentration of creatine kinase isoenzyme MB compared in serum after acute myocardial infarction

We compared three current methods (immunoinhibition, "Isomune-CK" immunoprecipitation, and the Tandem-E CKMB II immunoenzymometric assay) for determination of creatine kinase (CK; EC 2.7.3.2) isoenzyme MB in serum. Although results inter-correlated well, the immunoinhibition assay gave higher activity values. Atypical CK forms did not interfere with the immunoprecipitation and immunoenzymometric methods. In acute myocardial infarction the catalytic properties of CK decreased with the enzyme's age, as reflected by a steady increase in activation energy of the catalyzed reaction. In septicemia patients with very low CK and CK-MB catalytic activity, mean CK-MB mass concentration exceeded the upper reference limit, suggesting an increased rate of loss of activity concentration in these patients' sera. Because of the assay's lesser susceptibility to conformational changes at the active site of the enzyme, we suggest that measurement of CK-MB mass concentration is better suited for infarct sizing than measurement of catalytic activity.     

Development and performance of an enzyme immunoassay to detect creatine kinase isoenzyme MB activity using anti-mitochondrial creatine kinase monoclonal antibodies

Abstract

Objective: The MB fraction of creatine kinase (CK-MB) has long been used as a cardiac marker. It is known that the CK-MB immunoinhibition method lacks selectivity and accuracy, because the appearance of macro CK type 2, corresponding to mitochondrial creatine kinase (MtCK) in some patient serum may render CK-MB activity measured by conventional method abnormally high. Thus, to improve the specificity and accuracy of the CK-MB assay, we developed two types of monoclonal anti-MtCK antibodies against sarcomeric MtCK and ubiquitous MtCK, and present herein the performance of a new method using these antibodies. Material and methods: The performance of our test for detecting CK-MB activity was compared with other methods, and the range of CK-MB activities in normal human serum was investigated. Results: The two types of monoclonal antibodies developed by us were isoenzyme-specific to sMtCK or uMtCK. The correlation coefficients of our method and conventional method to electrophoresis were 0.973 and 0.873, respectively. The mean CK-MB activity in normal human serum by our method and the conventional method was 2.4 and 11.7 U/L, respectively. Thus, our data indicated that about 80% of CK-MB activity, determined using the conventional method, seems to correspond to the MtCK activity. Conclusion: Our method is novel in offering higher accuracy of measuring true CK-MB contents in human serum as compared to the conventional method. The possibility of accurately estimating CK-MB activity by our method which can inhibit MtCKs in healthy person and patient serum is likely to bring a break-through in clinical diagnostics.     

Assay of creatine kinase isoenzyme MB in serum with time-resolved immunofluorometry

We describe the first time-resolved immunofluorometric assay for creatine kinase (EC 2.7.3.2) isoenzyme MB (CK-MB) in serum. The assay is based on the formation of the complex: solid-phase anti-CK-MB-CK-MB-biotinylated anti-CK-BB-streptavidin-BCPDA-Eu3+, where anti-CK-MB and anti-CK-BB are monoclonal antibodies against the CK isoenzymes MB and BB, respectively, and BCPDA is the europium chelator 4,7-bis(chlorosulfophenyl)-1,10-phenanthroline-2,9-dicarboxylic acid. The solid-phase complex is fluorescent and is measured on the dry solid-phase (microtiter well) in a specially designed time-resolved fluorometer that uses laser excitation. The assay requires 25 microL of serum and is not affected by the presence of either CK-MM (up to 5000 micrograms/L) or CK-BB (up to 1000 micrograms/L) in the sample. Precision and accuracy indices for the assay were satisfactory.     

Interpretation of changes in the activity of creatine kinase MB isoenzyme in serum after coronary artery bypass grafting

Measurement of CK-MB in the sera of patients undergoing coronary artery bypass grafting shows a significant increase above pre-operative levels in all cases. However, the timing of the peak post-operative activity and its level allow criteria to be proposed which differentiate between two classes of patients: those with myocardial infarction or lesser myocardial damage, in whom peak activities are seen in specimens taken 21 hours after operation; and those in whom peak activities are found in specimens taken 4-7 hours after operation, presumably reflecting reversible myocardial changes. Activity peaks of more than 50 U/l occurring at 21 hours are considered to support a diagnosis of myocardial infarction.     

Differentiating muscle damage from myocardial injury by means of the serum creatine kinase (CK) isoenzyme MB mass measurement/total CK activity ratio

We immunoenzymometrically measured creatine kinase (CK) isoenzyme MB in extracts of myocardium and in homogenates of five different skeletal muscles. CK-MB concentrations in the former averaged 80.9 micrograms/g wet tissue; in the skeletal muscles it varied widely, being (e.g.) 25-fold greater in diaphragm than in psoas. CK-MB in skeletal muscles ranged from 0.9 to 44 ng/U of total CK; the mean for myocardium was 202 ng/U. In sera from 10 trauma and 36 burn patients without myocardial involvement, maximum ratios for CK-MB mass/total CK activity averaged 7 (SEM 1) ng/U and 18 (SEM 6) ng/U, respectively. Except for an infant (220 ng/U), the highest ratio we found for serum after muscular damage was 38 ng/U. In contrast, the mean maximum ratio determined in 23 cases of acute myocardial infarction exceeded 200 ng/U. Among seven determinations performed 8 to 32 h after onset of symptoms, each infarct patient demonstrated at least one ratio greater than or equal to 110 ng/U. Ratios observed after infarct were unrelated to treatment received during the acute phase. We propose a CK-MB/total CK ratio of 80 ng/U as the cutoff value for differentiating myocardial necrosis from muscular injury.     

Use of creatine kinase MB isoenzyme for diagnosing myocardial infarction when total creatine kinase activity is high

The usefulness of measuring creatine kinase MB isoenzyme for diagnosing myocardial infarction when activities of total creatine kinase are very high is unclear. We conducted a retrospective study in an urban hospital that serves a largely indigent population. We concentrated on 146 patients whose creatine kinase activity was greater than 1000 U/L (upper limit of normal: 165 U/L for women and 225 U/L for men), with MB isoenzyme greater than 10 U/L and less than 5% of total creatine kinase. The positive predictive value of MB isoenzyme (isoimmune method) values greater than 10 U/L was between 11.6% and 56.8% when the value for total creatine kinase exceeded 1000 U/L. Using different values (MB greater than 4% of total creatine kinase) as positive for myocardial infarction would have resulted in far fewer false-positives, but 10 cases of myocardial infarction would have been missed. The most appropriate cutoff value for MB isoenzyme in this population (total creatine kinase greater than 1000 U/L) was found to be greater than 2% of total creatine kinase.     

Evaluation of a commercial immunoenzymometric assay kit for creatine kinase MB isoenzyme determination using monoclonal antibodies

A simultaneous two-site immunoenzymometric assay for creatine kinase MB determination (Hybritech Tandem-E CK-MB) using monoclonal antibodies was evaluated and compared with cellulose acetate electrophoresis using fluorometric scanning densitometry. The assay has satisfactory precision (between-day analysis gives a coefficient of variation between 2.1 and 9.4%) and is not susceptible to interference by concentrations of creatine kinase MM up to 5000 micrograms/l (3400 U/l) and creatine kinase BB up to 1000 micrograms/l (1085 U/l). The upper limit of MB isoenzyme concentration in 250 apparently healthy people was 5.5 micrograms/l. Comparison between the immunoenzymometric assay (y) and electrophoresis (x) yielded the following linear regression equation: y = 0.37x + 1.9, with a correlation coefficient of 0.828. The characteristics of the temporal kinetics of MB isoenzyme, calculated by two methods, in 49 patients with acute myocardial infarction, were nearly identical in terms of the rate of creatine kinase MB release and the time at which the peak value is obtained, but not in terms of the rate of elimination of the isoenzyme. The fractional disappearance rate of MB isoenzyme from the circulation was significantly higher if calculated with Tandem-E results rather than with electrophoresis results (-0.035 vs -0.028, p less than 0.001). Whereas in the first day after infarction immunoenzymometric assay and electrophoresis had the same clinical sensitivity for identifying patients with acute myocardial infarction, in specimens collected more than 24 hours after the onset of the chest pain, the clinical sensitivity of the immunoenzymometric method was lower. Our results show that it is still premature to draw definitive clinical conclusions from the immunoassay results.     

Clinical and analytical evaluation of different methods for measurement of creatine kinase isoenzyme MB

We evaluated the clinical and analytical performance of the new immunochemiluminometric assay (ICMA; Ciba Corning) for measurement of creatine kinase isoenzyme MB (CK-MB), and compared it with three other methods: immunoradiometric assay (IRMA; International Immunoassay Labs); immunoinhibition assay (Seradyn); and an immunoinhibition/column method (Du Pont). Intra-test precision for all kits was good. We evaluated 32 patients' samples by all four methodologies. Only one of the four methods (aca, Du Pont) showed evidence of linearity. Efficiency in the diagnosis of myocardial injury in our study ranged from 53% (Seradyn) to 96% (Du Pont). We evaluated serial specimens from 20 separate patients by the IRMA and the ICMA to determine whether myocardial injury could be diagnosed earlier by the ICMA. In patients with acute myocardial infarction, the ICMA displayed positive values earlier and longer than the IRMA, suggesting that the ICMA is suited for screening for myocardial damage in hospitalized patients.     

Automated quantification of creatine kinase MB isoenzyme in serum by radial partition immunoassay, with use of the Stratus analyzer

We evaluated the analytical and clinical performances of a new radial partition immunoassay for measuring the mass concentration of creatine kinase (CK)-MB in serum. All pipetting, washes, incubations and data reduction were performed in 8 min by the Stratus (Dade) fluorometric analyzer. Within-assay and between-assay CVs were respectively 5.5% and 8.4% at 21 micrograms/L, and 4.2% and 3.4% at 48 micrograms/L. Assaying serial dilutions of serum samples with high CK-MB concentrations demonstrated excellent linearity. Results of the Stratus technique correlated well (n = 115, r = 0.98) with those of the Tandem-E CKMB II assay. There was no interference from hemolysis, bilirubin, rheumatoid factor, or added CK-MM (up to 3500 U/L); consequently, CK-MB can be determined in undiluted serum, even in the presence of high total CK activity. The mean CK-MB concentration in 105 blood donors was 1.9 (SD 1.3) micrograms/L. For seven myocardial infarction patients who received prompt fibrinolytic therapy, the mean CK-MB concentration was 4.5 (SD 1.8) micrograms/L at admission, and maximum concentrations, 119 (SD 94) micrograms/L, were recorded 16 h later. CK-MB returned to concentrations less than 10 micrograms/L within 72 h.     

Immunoenzymometric assay of creatine kinase with monoclonal antibodies to the MB isoenzyme compared with electrophoresis

We compare a "second-generation" immunoenzymometric assay (Tandem-E CKMB II) for creatine kinase (EC 2.7.3.2) MB with its electrophoretic (Beckman Paragon system) determination. In the former, two monoclonal antibodies are directed against the B and M subunits. We evaluated 502 samples from 253 patients. Precision, linearity, and analytical recovery for both assays were excellent. The two methods correlated well (r = 0.936). The reference interval for individuals with no suspected cardiac disorder was 0-6.0 micrograms/L; that for non-infarct patients was 0-18.0 micrograms/L. Peak CK-MB values determined by the two assays agreed for 95% of the patients, in terms of exceeding the normal reference interval or not. Diagnostic efficiencies were 86% (Tandem) and 88% (electrophoresis). The immunoenzymometric assay showed no cross reaction with other CK isoenzymes. Both assay methods performed well in detecting CK-MB, although there were some false positives by both methods, as judged from electrocardiographic results. When total CK for the Tandem assay exceeds 2000 U/L, we recommend calculation of a ratio (CK-MB, micrograms/L:total CK, U/L).