Acute coronary syndrome (ACS), is one of the leading causes of morbidity and mortality worldwide . Many of these patients have normal or nonspecific electrocardiographic changes, as well as normal admission CK–MB and troponin levels. That is why having a cardiac biomarker that could detect myocardial ischemia before the occurrence of myocardial necrosis would be of great value and impact on management of such patients in chest pain and coronary care units . Ischemia modified albumin (IMA), a serum biomarker, is an albumin that has an altered capacity to bind metal ions such as cobalt (Co), copper (Cu) and nickel (Ni) in its N-terminus. It is produced and can be measured in the serum during acute ischemic events . These changes are presumed to be related to the production of reactive oxygen species (ROS) during ischemia and reperfusion, hypoxia, and acidosis. Therefore, IMA can be used to detect myocardial ischemia in patients presenting with suspected acute coronary syndrome .
Aim of work
To study the IMA level in patients presenting with non-ST segment elevation acute coronary syndromes and to determine its diagnostic and prognostic value.
Patients and methods
This study was prospective observational study, conducted on 50 patients with a mean age of 54.7 ± 9 yrs. Of these 30 patients presented to the critical care department of Cairo University with typical chest pain of less than 4 h duration and were diagnosed as having non-ST elevation acute coronary syndrome during the period of September 2012 to January 2013 were included in the study. Patients were then segregated according to troponin level into unstable angina (UA) (gp 1), NSTEMI (gp 2). Another 20 patients referred to the catheter laboratory for suspected coronary artery disease and diagnosed as having normal coronaries served as control (gp 3). All patients gave informed written consents and the protocol was approved by the local ethical committee and in accordance with the declaration of Helsinki. All patients were subjected to all of the following: full history taking, routine general and systemic examination, twelve-lead ECG, routine laboratory tests, cardiac biomarkers including qualitative troponin that were done upon admission and serially every 8 h for 24 h. For all patients TIMI risk score was estimated: (age 65 year, three or more risk factors of CAD, known CAD, ASA use in the past 7 days, Severe angina: two episodes within 24 h, ST changes > 0.5 mm, and +ve cardiac marker). If any of the previous items were present it is graded as one, absent graded as zero with maximum score 7 and minimum of 0. Standard 2D, M mode, pulsed and color Doppler using parasternal and apical views echocardiographic examination. For all patients plasma IMA level was measured upon admission and on the day of coronary angiography for control group using Uscn, Inc. enzyme-linked immunosorbent Assay (ELISA).
Material collection and stability
Plasma samples were collected using EDTA or heparin as an anticoagulant. Samples centrifuged for 15 min at 1000×g at 2–8 °C within 30 min of sample collection. Samples were stored at −20 °C to avoid loss of bioactivity and contamination.
The microtiter plate provided in this kit has been pre-coated with a monoclonal antibody specific to IMA. Standards or samples were then added to the appropriate microtiter plate wells with a biotin-conjugated polyclonal antibody preparation specific for IMA. Next, Avidin conjugated to horseradish peroxidase (HRP) was added to each microplate well and incubated. Then a tetramethylbenzidine (TMB) substrate solution was added to each well. Only those wells that contain IMA, biotin-conjugated antibody and enzyme-conjugated Avidin will exhibit a change in color. The enzyme–substrate reaction was terminated by the addition of a sulfuric acid solution and the color change was measured spectrophotometrically at a wavelength of 450 nm ± 10 nm. The concentration of IMA in the samples was then determined by comparing the O.D. of the samples to the standard curve.
All patients included in the study were subjected to pre-discharge coronary angiography. Coronary angiogram was undertaken by the percutaneous trans femoral or trans radial approach and all images were recorded digitally. Coronary angiograms were scored visually into a severity score (0–3) which defined the number of vessels with a luminal stenosis of 50% (for right, left anterior descending, and circumflex arteries or its main branch). Then the severity and extent of CAD were graded using a modified Gensini score (MGS). The most severe stenosis in each of the three coronaries and in the left main was graded from 0 to 6; 0 no stenosis; 1, 1–29% stenosis; 2, 30–49% stenosis; 3, 50–69% stenosis; 4, 70–89% stenosis; 5, 90–99%; stenosis 6, 100% occlusion and summed to yield a score of 0–24 . During hospital stay and 2 months after discharge, patients were followed to detect the occurrence of any major adverse cardiac events (MACE): (1) Recurring myocardial ischemia, translated by the reappearance of angina or acute myocardial infarction (STEMI or NON STEMI). (2) Left sided heart failure defined by signs and symptoms of pulmonary congestion, requiring the use of specific therapy, such as diuretics, vasodilators, or inotropic support. (3) Cardiogenic shock defined as systolic blood pressure <90 mm Hg for more than 30 min, requiring the use of vasopressors or the development of metabolic acidosis. (4) Arrhythmias requiring pharmacological treatment, electrical cardioversion or use of pacemaker. (5) Death .
All patients were managed according to the latest European guidelines
Any patients who had any of the following: age <18 years, history of recent cerebrovascular accidents within 1 month of admission, advanced peripheral vascular disease, acute limb ischemia, recent history of chest trauma, serum creatinine >1.5 mg/dl, or abnormal albumin level i.e. outside the reference range of 3.5–5 mg/dl were excluded from the study.
Computer software package SPSS 15.0 was used in the analysis. For quantitative variables, mean/median (as a measure of central tendency), standard deviation/range, minimum, and maximum (as measures of variability) were presented. Frequency and percentages were presented for qualitative variables. Mann–Whitney, Kruskal–Wallis and ANOVA tests were used to estimate differences in quantitative variables. Chi-square and Fisher's exact tests were used to estimate differences in qualitative variables. Correlation to estimate association between quantitative variables was presented in the form of a correlation coefficient and its significance. A probability value (P value) less than 0.05 was considered significant. Receiver operating characteristic (ROC) curves were plotted to determine the best diagnostic and prognostic cut-off values for IMA.
During the period of September 2012 to January 2013, 30 patients admitted to the critical care department of Cairo university and diagnosed as having non ST elevation acute coronary syndrome were enrolled and furtherly divided according to the troponin level into: unstable angina group (gp 1), and NSTEMI group (gp 2). Another 20 patients referred to the cath laboratory because of suspected coronary artery disease and their results revealed normal coronary angiogram served as the control group (gp3). Patients' demographic and general characteristics on admission are shown in Table 1.
As shown in Table 1, all patients in the study groups were comparable to each other, with no statistically significant difference regarding their demographic and general characteristics data.
IMA level in different studied groups
The IMA level was significantly higher in group 1 and group 2 compared to group 3 patients. Moreover, patients in group 1 had higher levels of IMA compared to group 2 Table 2.
Diagnostic ability of IMA
To detect the ability of IMA to diagnose patients presenting with acute coronary syndrome, all patients in both groups 1 and 2 were compared to patients in the control group. It was found that IMA levels were significantly higher in patients compared to controls for whom diagnostic angiography revealed normal coronaries (Table 3).
Receiver operator characteristics (ROC) curve was calculated to detect the diagnostic ability of IMA in patients presenting with suspected acute coronary syndrome in the coronary care unit. The optimal cutoff value was 4 ng/ml, and this cutoff value had a sensitivity of 71%, specificity of 63.1%, positive predictive value 75.9% and negative predictive value 57.1% with diagnostic accuracy of 71.1% (Fig. 1).
IMA level and other cardiac biomarkers
There was a significant positive correlation between IMA level, CK, and CK–MB level of the study patients (Table 4).
Nonetheless, the mean level of IMA was statistically significantly higher in troponin positive patients compared to troponin negative patients (P value = 0.026) (Table 5).
IMA level and TIMI risk score
There was a highly statistically significant positive correlation between IMA level and TIMI risk scores of the study patients (r = 0.442, P = 0.001) (Table 6).
IMA level and angiographic findings
There was a significant positive correlation between IMA level and number of vessels affected of the studied patients with positive coronary angiography (patients in groups 1 and 2) (r = 0.443, P = 0.049) (Table 7).
However, there was no significant correlation between IMA level and degree of vessel stenosis of the study patients using MGS (r = 0.070, P = 0.714) (Table 8).
Prognostic ability of IMA
During the follow-up period (2 months), among 48 patients who survived, two suffered from benign arrythmia, two suffered from malignant arrythmia, two had recurrent chest pain and one patient was subjected to surgical revascularization. The mean level of IMA was higher in morbid patients when compared with the others, but statistically insignificant (P value = 0.735) (Table 9).
A receiver operator characteristics (ROC) curve was calculated for the use of IMA level as a predictor of morbidity and mortality. The optimal cutoff value for IMA level that could be used to predict morbidity and mortality was 9.65 ng/ml. This cutoff value gave a sensitivity of 66.6%, specificity of 88.6%, positive predictive value of 44.4%, negative predictive value of 95.1% with an overall accuracy of 86% (Fig. 2).
The ability to detect myocardial ischemia before myocardial necrosis would allow earlier and more accurate management decisions for patients admitted with suspected acute coronary syndrome than is currently possible based on serum troponin, CK–MB, or myoglobin levels, especially in normal ECG patients. Previous studies have demonstrated that IMA levels may be elevated and even precede cardiac troponin elevations in patients with cardiac ischemia . This early prediction by a biochemical marker of ischemia is important, as it may improve the ability to stratify acute chest pain patients and guide therapeutic decisions. Thus, the aim of this study was to test the accuracy of IMA to diagnose non-ST segment elevation acute coronary syndromes and to determine if the IMA level correlates with morbidity and mortality. This study showed that IMA levels were significantly higher in ischemic patients compared to the control group (10.84 ± 13.78 ng/ml vs 3.02 ± 3.47 ng/ml, P value: 0.011) and positively correlated with other cardiac biomarkers (CK, CK–MB, and qualitative troponin). Importantly, the IMA level was significantly higher in unstable angina patients compared to non ST-segment elevation patients (14.11 ± 16.45 vs 8.70 ± 11.7 ng/ml, P value: <0.05), implying that elevation of IMA is related to myocardial ischemia and not related to myocardial necrosis. Sinha et al. used the albumin cobalt binding (ACB) test to measure IMA level and reported that the sensitivity of IMA at presentation for an ischemic origin of chest pain was 82%, compared with 20% of cTnT, indicating that elevation of IMA was related to myocardial ischemia rather than myocardial injury . Bar-Or et al. reported the same findings, as they found that IMA elevation occurred within minutes of myocardial ischemia and remained for several hours later before the development of myocardial necrosis, as evidenced by normal CK, CK–MB, myoglobin and troponin levels . To the best of our knowledge, none of the previous studies used ELISA technique to measure IMA level in ACS patients, also there is no method to convert units measured by ACB test to nanograms; the unit used to measure IMA level using ELISA technique. In 2011, Amit et al. reported that there was a significant increase in IMA level in patients with acute ischemic stroke at admission, 24 h, 48 h and 144 h, respectively, when compared with controls . Moreover, Turedi et al. reported that IMA level is significantly higher in patients with pulmonary embolism compared to controls  This means that IMA is a sensitive test to diagnose ischemic events but not specific to myocardial ischemia. So it can be used to diagnose acute coronary syndromes only after exclusion of other ischemic conditions. In the present study a cut-off value of 4 ng/ml had a sensitivity of 71%, specificity of 63.1%, positive predictive value 75.9% and negative predictive value 57.1%. This result indicates that this cut-off value of IMA may improve our ability to rule out patients who do not have acute coronary syndrome. In contrast, in 2010 Richard Ming-Hui Lin et al. failed to find any diagnostic value for adding IMA level measurement to troponin measurement in patients presenting with suspected acute coronary syndrome . The discrepancy between both studies may be related to the difference of the study population. While all their studied patients were admitted with suspected ACS, two-fifth of our studied patients served as control. In this study, there was a highly statistical significant positive correlation between IMA levels and TIMI risk score of the study patients (r = 0.442, P = 0.001). Also, there was significant positive correlation between IMA levels and the extent of the coronary artery disease, defined by number of vessels affected in ischemic patients (r = 0.443, P = 0.049) but not the severity of the disease as defined by MGS (r = 0.070, P = 0.714). As a predictor of mortality, IMA at a level of 9.65 ng/ml had a sensitivity of 66.6% and specificity of 88.6% (PPV 44.4%, NPV 95.1% and diagnostic accuracy of 86%). Although the mean level of IMA was higher in morbid patients (during the follow-up period) when compared with non-morbid patients, this difference was statistically insignificant (P value >0.05). Contrary to our results, the results published by Bali et al. in 2008 reported that the median IMA level was significantly higher in patients with MACE during hospitalization (P = 0.007) and at 1 year (P < 0.001) . Yet the lack of this statistical significance in our study could be explained by the small sample size and a shorter follow-up period compared to the study of Bali et al. (2 months vs. 1 year follow-up).
Serum IMA is a useful marker to rule out non-ST segment elevation acute coronary syndrome, and segregate UA from NSTEMI. It correlates well with TIMI score and extent of CAD, but does not correlate with short-term outcome.
In addition to the small number of sample size, the accuracy of the method used to measure IMA level in our study, ELISA technique, has not been tested before.
Conflict of interest statement
No conflict of interest for the authors.
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