Autoantibodies against Cardiac Troponins
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《新英格兰医药杂志》
To the Editor: Cardiac troponins are preferred markers of tissue injury in acute coronary syndromes. Their specificity and the fact that only very minute amounts or none are normally found in the circulation provide high clinical sensitivity and specificity even when cardiac lesions are small. The importance of cardiac troponins in identifying high-risk patients to receive active intervention is widely recognized.1
Troponins present substantial challenges with respect to immunoassays, however. They occur as various complexes or fragments and show other forms of heterogeneity.2 In addition, we recently reported3 that serum from many patients contains an interfering factor that prevents the recognition of exogenous and endogenous cardiac troponin I. In such patients with low recovery of added cardiac troponin I, release of cardiac troponin I from injured tissue is detected poorly or only after an interval that may last more than 12 hours. This is especially true of immunoassays, in which the antibodies recognize the stable central portion (mid-fragment) of cardiac troponin I (amino acid residues 30 to 110), the analytic approach currently favored.4
We have found that this interference is due to antibodies to cardiac troponin I or its circulating complexes. We tested serum from 10 patients with low recovery of added cardiac troponin I (0 to 27 percent; median, 10.6 percent) and 10 with normal recovery (91 to 101 percent) for the presence of autoantibodies to cardiac troponin I. Samples were incubated in microtiter wells precoated with cardiac troponin I complexes. The amounts of immunoglobulin were quantified with labeled mouse antihuman antibodies. The box-plot diagram in Figure 1A shows a significant difference in signals between patients with low recovery and those with normal recovery.
Figure 1. Antibodies to Cardiac Troponin I According to the Level of Recovery of Added Cardiac Troponin I Standard (Panel A) and Measurements of Cardiac Troponin I with Two Different Assays in a Patient with an Acute Coronary Syndrome (Panel B).
Serum samples from 10 patients with low recovery of added cardiac troponin I (LR) and 10 with normal recovery (NR) were tested for the presence of antibodies against cardiac troponin I with the use of labeled mouse antihuman antibodies. As shown in the box-plot diagram in Panel A, the difference was statistically significant (P=0.002 by the Mann–Whitney test). In this diagram, the box represents the interquartile range, the horizontal line inside the box the median, and the I bars the 10th and 90th percentiles of the values. Panel B shows cardiac troponin I concentrations in three serum samples from a low-recovery patient with an acute coronary syndrome, as measured by a conventional mid-fragment assay (Aio-cTnI, Innotrac Diagnostics; solid diamonds) and by a novel assay also incorporating antibodies outside the stable mid-fragment (open squares). With the conventional assay, only the 24-hour sample had detectable cardiac troponin I. With the novel assay design, all three samples were clearly positive.
The consequence of the presence of autoantibodies with regard to measurements of cardiac troponin I is illustrated in Figure 1B, in which two differently configured assays were used in the follow-up of a low-recovery patient with an acute coronary syndrome. The assay based on two mid-fragment antibodies had negative results for both the sample obtained at admission and that obtained after 6 to 12 hours, whereas an assay that also included antibodies outside the mid-fragment was positive in all three samples tested.
We conclude that autoantibodies to cardiac troponin I can hamper the triage of patients with acute coronary syndrome by causing delays in the detection of cardiac troponin I. At exceptionally high titers, these antibodies may totally mask the release of small amounts of cardiac troponin I. Patients with low recovery are commonly identified with mid-fragment cardiac troponin I assays, and about 10 percent have recovery of less than 50 percent.3 To avoid misclassification of patients with myocardial infarction or delays in identifying them, assays should be made sensitive enough to detect a substantially blocked signal. Preferably, alternative assays should be used to recognize autoantibody complexes. Such improvements may make it possible to use cardiac troponin I not only as a specific marker, but also as an early marker of cardiac injury.
Susann Eriksson, M.Sc.
Jukka Hellman, Ph.D.
Kim Pettersson, Ph.D.
University of Turku
FIN-20520 Turku, Finland
kim.pettersson@utu.fi
Dr. Pettersson reports being a shareholder and board member of Innotrac Diagnostics Oy (Turku, Finland), the manufacturer of the Aio cardiac troponin I kit. The novel assay mentioned in the letter is an investigational assay, but the same basic concept is now being manufactured by Innotrac Diagnostics.
References
Antman EM, Tanasijevic MJ, Thompson B, et al. Cardiac-specific troponin I levels to predict the risk of mortality in patients with acute coronary syndromes. N Engl J Med 1996;335:1342-1349.
Katrukha A, Bereznikova A, Filatov V, Esakova T. Biochemical factors influencing measurement of cardiac troponin I in serum. Clin Chem Lab Med 1999;37:1091-1095.
Eriksson S, Junikka M, Laitinen P, Majamaa-Voltti K, Alfthan H, Pettersson K. Negative interference in cardiac troponin I immunoassays from a frequently occurring serum and plasma component. Clin Chem 2003;49:1095-1104.
Panteghini MM, Gerhardt W, Apple FS, Dati F, Ravkilde J, Wu AH. Quality specifications for cardiac troponin assays. Clin Chem Lab Med 2001;39:175-179.
Troponins present substantial challenges with respect to immunoassays, however. They occur as various complexes or fragments and show other forms of heterogeneity.2 In addition, we recently reported3 that serum from many patients contains an interfering factor that prevents the recognition of exogenous and endogenous cardiac troponin I. In such patients with low recovery of added cardiac troponin I, release of cardiac troponin I from injured tissue is detected poorly or only after an interval that may last more than 12 hours. This is especially true of immunoassays, in which the antibodies recognize the stable central portion (mid-fragment) of cardiac troponin I (amino acid residues 30 to 110), the analytic approach currently favored.4
We have found that this interference is due to antibodies to cardiac troponin I or its circulating complexes. We tested serum from 10 patients with low recovery of added cardiac troponin I (0 to 27 percent; median, 10.6 percent) and 10 with normal recovery (91 to 101 percent) for the presence of autoantibodies to cardiac troponin I. Samples were incubated in microtiter wells precoated with cardiac troponin I complexes. The amounts of immunoglobulin were quantified with labeled mouse antihuman antibodies. The box-plot diagram in Figure 1A shows a significant difference in signals between patients with low recovery and those with normal recovery.
Figure 1. Antibodies to Cardiac Troponin I According to the Level of Recovery of Added Cardiac Troponin I Standard (Panel A) and Measurements of Cardiac Troponin I with Two Different Assays in a Patient with an Acute Coronary Syndrome (Panel B).
Serum samples from 10 patients with low recovery of added cardiac troponin I (LR) and 10 with normal recovery (NR) were tested for the presence of antibodies against cardiac troponin I with the use of labeled mouse antihuman antibodies. As shown in the box-plot diagram in Panel A, the difference was statistically significant (P=0.002 by the Mann–Whitney test). In this diagram, the box represents the interquartile range, the horizontal line inside the box the median, and the I bars the 10th and 90th percentiles of the values. Panel B shows cardiac troponin I concentrations in three serum samples from a low-recovery patient with an acute coronary syndrome, as measured by a conventional mid-fragment assay (Aio-cTnI, Innotrac Diagnostics; solid diamonds) and by a novel assay also incorporating antibodies outside the stable mid-fragment (open squares). With the conventional assay, only the 24-hour sample had detectable cardiac troponin I. With the novel assay design, all three samples were clearly positive.
The consequence of the presence of autoantibodies with regard to measurements of cardiac troponin I is illustrated in Figure 1B, in which two differently configured assays were used in the follow-up of a low-recovery patient with an acute coronary syndrome. The assay based on two mid-fragment antibodies had negative results for both the sample obtained at admission and that obtained after 6 to 12 hours, whereas an assay that also included antibodies outside the mid-fragment was positive in all three samples tested.
We conclude that autoantibodies to cardiac troponin I can hamper the triage of patients with acute coronary syndrome by causing delays in the detection of cardiac troponin I. At exceptionally high titers, these antibodies may totally mask the release of small amounts of cardiac troponin I. Patients with low recovery are commonly identified with mid-fragment cardiac troponin I assays, and about 10 percent have recovery of less than 50 percent.3 To avoid misclassification of patients with myocardial infarction or delays in identifying them, assays should be made sensitive enough to detect a substantially blocked signal. Preferably, alternative assays should be used to recognize autoantibody complexes. Such improvements may make it possible to use cardiac troponin I not only as a specific marker, but also as an early marker of cardiac injury.
Susann Eriksson, M.Sc.
Jukka Hellman, Ph.D.
Kim Pettersson, Ph.D.
University of Turku
FIN-20520 Turku, Finland
kim.pettersson@utu.fi
Dr. Pettersson reports being a shareholder and board member of Innotrac Diagnostics Oy (Turku, Finland), the manufacturer of the Aio cardiac troponin I kit. The novel assay mentioned in the letter is an investigational assay, but the same basic concept is now being manufactured by Innotrac Diagnostics.
References
Antman EM, Tanasijevic MJ, Thompson B, et al. Cardiac-specific troponin I levels to predict the risk of mortality in patients with acute coronary syndromes. N Engl J Med 1996;335:1342-1349.
Katrukha A, Bereznikova A, Filatov V, Esakova T. Biochemical factors influencing measurement of cardiac troponin I in serum. Clin Chem Lab Med 1999;37:1091-1095.
Eriksson S, Junikka M, Laitinen P, Majamaa-Voltti K, Alfthan H, Pettersson K. Negative interference in cardiac troponin I immunoassays from a frequently occurring serum and plasma component. Clin Chem 2003;49:1095-1104.
Panteghini MM, Gerhardt W, Apple FS, Dati F, Ravkilde J, Wu AH. Quality specifications for cardiac troponin assays. Clin Chem Lab Med 2001;39:175-179.