Prognostic usefulness of marginal troponin T elevation

Frequency of Multivessel Severe Coronary Artery Disease in Patients With Non-ST Segment Elevation Myocardial Infarction Having Markedly Raised Cardiac Troponin T

Introduction

Non-ST segment elevation myocardial infarction (NSTEMI) is becoming more common than ST segment elevation myocardial infarction (STEMI) and data regarding presence of underlying multivessel coronary artery disease (MVCAD) in these patients is consistent in locoregional population that leads to lethal delays in proper management. Therefore, in the current study, we aimed to evaluate the frequency of MVCAD in NSTEMI with markedly raised troponin T levels. This will help to identify patients that should be labeled as high risk and must be referred for coronary revascularization on priority basis, so that clinical outcomes can be improved in these patients.

Methods

This cross-sectional research study was carried out at Chaudhary Pervaiz Elahi Institute of Cardiology, Multan over a period of one year. A total of 326 patients with history of chest discomfort within past 48 hours of presentation or angina equivalent symptoms and cardiac troponin T more than 500 ng/l were included in the study. Coronary angiography was done within 72 hours of same hospital admission. The outcome variable i.e. MVCAD was determined.

Results

Mean age of patients was 50.74 ± 7.75 years with range of 30 to 60 years. MVCAD was found in 107 (32.82%) patients, whilst there was no MVCAD in 219 (67.18%) patients. Moreover, no significant association of MVCAD was noted with age or smoking.

Conclusion

We found presence of MVCAD in a considerable number of patients presenting with NSTEMI. The key to detect the underlying presence of MVCAD in these patients is lifted troponin T levels. Therefore, we conclude that any patient with elevated troponin T levels, even in the absence of ST segment elevation, should undergo cardiac catheterization to detect presence of MVCAD as this subset of patients can benefit from early revascularization including coronary artery bypass graft (CABG) surgery.

Early invasive strategy and outcomes of non-ST-elevation acute coronary syndrome patients: is time really the major determinant?

In non-ST-elevation acute coronary syndromes (ACS), an early invasive strategy is recommended for middle/high-risk patients; however, the optimal timing for coronary angiography is still debated. The aim of this study was to evaluate the prognostic implications of the time of angiography in ACS patients treated in accord with an early invasive strategy. We analyzed the relationship between the time of angiography and outcomes at follow-up in 517 ACS patients, of whom 482 were revascularized with percutaneous coronary intervention (PCI) (86.9%) or coronary artery by-pass graft (13.1%). We also evaluated the influence of clinical, biohumoral and angiographic variables on the patients' outcomes at follow-up. Among patients submitted to angiography at different time intervals from both hospital admission and symptom onset, significant differences neither in mortality nor in cardiac ischemic events at follow-up were observed. At univariate analysis, complete versus partial revascularization, longer hospital stay, higher TIMI risk score, diabetes mellitus, higher discharge creatinine and admission anemia were associated with mortality and cardiac ischemic events at follow-up; a lower left ventricular ejection fraction was associated with mortality; higher peak troponin I and previous PCI were associated with cardiac ischemic events at follow-up. At multivariate analysis longer hospital stay, higher discharge creatinine levels, and previous PCI were independent predictors of cardiac ischemic events at follow-up. Our evaluation in ACS patients treated with an early invasive strategy does not support the concept that angiography should be performed as soon as possible after symptom onset or hospital admission. Rather, an unfavorable long-term outcome is influenced principally by the clinical complexity of patients.

Prognostic significance of cardiac troponin I levels in hospitalized patients presenting with supraventricular tachycardia

Although cardiac troponin I (cTnI) elevation in patients presenting to the hospital with supraventricular tachycardia (SVT) is well recognized, the prevalence, predictors, and prognostic significance of cTnI elevation associated with SVT presentation are not known. We screened records of all patients presenting to 2 hospitals over a 4-year period with the diagnosis of SVT confirmed by 12-lead electrocardiogram, and who had at least 1 measured cTnI level and at least 1 year of follow-up after discharge. The primary endpoint was the occurrence of 1 of the following outcomes: death, myocardial infarction, or cardiovascular rehospitalization. Seventy-eight patients met the study criteria (54% female; mean age, 62.2 +/- 15.8 yr), and 29 patients (37.2%) had an elevated cTnI level of > or =0.06 ng/mL (range, 0.06-7.78 ng/mL). Univariate predictors of elevated cTnI included left ventricular ejection fraction (LVEF) <50%, renal dysfunction, ST-segment depression or left bundle branch block on the electrocardiogram, and moderate or severe regurgitation of any cardiac valve. Predictors of elevated cTnI after multivariate analysis included peak heart rate during SVT (per 15 bpm) (odds ratio [OR], 1.58; 95% confidence interval [CI], 1.01-2.46; p = 0.04) and LVEF <50% (OR, 6.12; 95% CI, 1.40-26.7; p = 0.02). After multivariable adjustment, the presence of elevated cTnI with SVT was associated with increased risk of the primary endpoint of death, myocardial infarction, or cardiovascular rehospitalization (hazard ratio [HR], 3.67; 95% CI, 1.22-11.1; p = 0.02). Mild elevation of cTnI is common in patients presenting to the hospital with SVT, and is associated with increased risk of future cardiovascular events. Further study is needed to determine the mechanisms of SVT-related cTnI elevation and its association with elevated cardiovascular risk.

Outcomes associated with small changes in normal-range cardiac markers

INTRODUCTION

Troponin concentrations rising above an institutional cutpoint are used to define acute myocardial necrosis, yet it is uncertain what outcomes are associated with fluctuations in troponin that do not exceed this level. We evaluate the association between troponin fluctuations below an institutional upper limit of normal and acute coronary syndrome (ACS).

MATERIALS AND METHODS

This was a post hoc analysis of the Internet tracking registry of ACS (i*trACS), which describes patients presenting to emergency departments (EDs) with suspected ACS across the spectrum of risk. Patients were included in this registry if they were at least 18 years old and had suspected ACS at the time of their ED visit. Inclusions in this analysis required that patients had at least 1 cardiac marker (creatine kinase-MB [CK-MB], troponin T, or troponin I) drawn twice within 6 hours of presentation, with both measures being below the institution's upper limit of normal. A marker change was defined as either an increase or decrease that exceeded 15% of the institutional upper limit of normal. Acute coronary syndrome was defined as a positive stress test, documented myocardial infarction, coronary revascularization, or death within 30 days of their ED admission.

RESULTS

Of 17,713 patient visits, 2162 met inclusion and exclusion criteria. There were 1872 patient visits with 2 troponin results and 1312 with 2 CK-MB results. Patient visits with increasing troponin had increased odds of ACS compared with those with stable troponin levels (odds ratio, 3.6; 95% confidence interval, 1.4-9.2). Changing CK-MB and decreasing troponin were not associated with increased odds of ACS.

CONCLUSIONS

Small increases in troponin concentration below the upper limit of normal are associated with increased odds of ACS.

Diagnostic accuracy of clinical prediction rules to exclude acute coronary syndrome in the emergency department setting: a systematic review

OBJECTIVE

We sought to determine the diagnostic accuracy of clinical prediction rules to exclude acute coronary syndrome (ACS) in the emergency department (ED) setting.

METHODS

We searched MEDLINE, EMBASE, Web of Science and the Cochrane Database of Systematic Reviews. We contacted content experts to identify additional articles for review. Reference lists of included studies were hand searched. We selected articles for review based on the following criteria: 1) enrolled consecutive ED patients; 2) incorporated variables from the history or physical examination, electrocardiogram and cardiac biomarkers; 3) did not incorporate cardiac stress testing or coronary angiography into prediction rule; 4) based on original research; 5) prospectively derived or validated; 6) did not require use of a computer; and 7) reported sufficient data to construct a 2 x 2 contingency table. We assessed study quality and extracted data independently and in duplicate using a standardized data extraction form.

RESULTS

Eight studies met inclusion criteria, encompassing 7937 patients. None of the studies verified the prediction rule with a reference standard on all or a random sample of patients. Six studies did not report blinding prediction rule assessors to reference standard results, and vice versa. Three prediction rules were prospectively validated. Sensitivities and specificities ranged from 94% to 100% and 13% to 57%, and positive and negative likelihood ratios from 1.1 to 2.2 and 0.01 to 0.17, respectively.

CONCLUSION

Current prediction rules for ACS have substantial methodological limitations and have not been successfully implemented in the clinical setting. Future methodologically sound studies are needed to guide clinical practice.

Influence of renal function on the usefulness of N-terminal pro-B-type natriuretic peptide as a prognostic cardiac risk marker in patients undergoing noncardiac vascular surgery

N-terminal pro-B-type natriuretic peptide (NT-pro-BNP) is related to stress-induced myocardial ischemia and/or volume overload, both common in patients with renal dysfunction. This might compromise the prognostic usefulness of NT-pro-BNP in patients with renal impairment before vascular surgery. We assessed the prognostic value of NT-pro-BNP in the entire strata of renal function. In 356 patients (median age 69 years, 77% men), cardiac history, glomerular filtration rate (GFR, ml/min/1.73 m(2)), and NT-pro-BNP level (pg/ml) were assessed preoperatively. Troponin T and electrocardiography were assessed postoperatively on days 1, 3, 7, and 30. The end point was the composite of cardiovascular death, Q-wave myocardial infarction, and troponin T release. Multivariate analysis was used to evaluate the interaction between GFR, NT-pro-BNP and their association with postoperative outcome. Median GFR was 78 ml/min/1.73 m(2) and the median concentration of NT-pro-BNP was 197 pg/ml. The end point was reached in 64 patients (18%); cardiac death occurred in 7 (2.0%), Q-wave myocardial infarction in 34 (9.6%), and non-Q-wave myocardial infarction in 23 (6.5%). After adjustment for confounders, NT-pro-BNP levels and GFR remained significantly associated with the end point (p = 0.005). The prognostic value of NT-pro-BNP was most pronounced in patients with GFR > or =90 (odds ratio [OR] 1.18, 95% confidence interval [CI] 0.80 to 1.76) compared with patients with GFR 60 to 89 (OR 1.04, 95% CI 1.002 to 1.07), and with GFR 30 to 59 (OR 1.12, 95% CI 1.03 to 1.21). In patients with GFR <30 ml/min/1.73 m(2), NT-pro-BNP levels have no prognostic value (OR 1.00, 95% CI 0.99 to 1.01). In conclusion, the discriminative value of NT-pro-BNP is most pronounced in patients with GFR > or =90 ml/min/1.73 m(2) and has no prognostic value in patients with GFR <30 ml/min/1.73 m(2).

Use and Misuse of Cardiac Troponins in Clinical Practice

Cardiac troponins are very sensitive and specific markers of myocardial injury. Elevated troponin levels in the setting of acute coronary syndrome are diagnostic of acute myocardial infarction and provide guidance to clinicians with regard to appropriate use of intensive medical and revascularization therapies. However, elevated troponin levels are commonly seen in several noncoronary ischemia presentations and create considerable confusion among clinicians in these settings. In this review article, we discuss the utility of troponins in various clinical settings and present a "common sense" approach to interpreting troponin elevation outside the setting of acute coronary syndrome.

The use of biomarkers for the evaluation and treatment of patients with acute coronary syndromes

The advent of inexpensive, highly accurate, and predictive markers of myocardial injury, inflammation, and hemodynamic stability has revolutionized the evaluation and treatment of patients who have acute coronary syndromes (ACSs). These blood biomarkers require small sample volumes, can be run expeditiously, and provide important information concerning the diagnosis, risk stratification, and treatment of these patients. To understand the use of these markers, one must have some knowledge about what elevations in these markers imply, how they have to be collected and measured to provide reliable information, when to suspect analytic confounds, and what the key values are that impart the diagnostic, prognostic, and therapeutic information. This article discusses these issues, emphasizing what clinicians must know for optimal test use, and then addresses the practical use of these markers in patients who have ACS.

Impact of glomerular filtration rate on minor troponin T elevations for risk assessment in patients undergoing operation for abdominal aortic aneurysm or lower extremity arterial obstruction

Debate surrounds the impact of renal function on the prognostic value of minor troponin T release in vascular surgery patients. The objective of this study was to assess the long-term prognostic value of minor degrees of troponin T release in patients who undergo major vascular surgery, especially those with concomitant renal dysfunction. Survivors of major noncardiac vascular surgery (n = 558) were preoperatively screened for cardiac risk factors and renal function. Serial troponin T was measured on days 1, 3, and 7 after surgery, using a threshold of 0.03 ng/ml. All-cause mortality and major adverse cardiac events (MACEs) were noted during follow-up (mean 3.5 +/- 2.0 years). Minor (0.03 to 0.09 ng/ml) and major (> or =0.1 ng/ml) release of troponin T was observed in 5% and 8%, respectively. During follow-up, 21% of the patients died and 15% experienced MACEs. After adjustment for the estimated glomerular filtration rate, patients with minor and major troponin T release were at comparable increased risk for late mortality (hazard ratio [HR] 3.43, 95% confidence interval [CI] 1.79 to 6.58, and HR 3.72, 95% CI 2.37 to 5.85, respectively), and MACEs (HR 5.47, 95% CI 2.60 to 11.48, and HR 6.32, 95% CI 3.82 to 10.48, respectively) compared with patients with troponin T release <0.03 ng/ml. Tests for heterogeneity revealed that minor and major troponin T release have prognostic value across the entire spectrum of renal function. In conclusion, marginal elevations of troponin T strongly predict late mortality and MACEs after major vascular surgery, irrespective of renal function. A currently underestimated high-risk subgroup of patients may be identified using a lower troponin T threshold.

A Model for Troponin I as a Quantitative Predictor of In-Hospital Mortality

OBJECTIVES

We evaluated log-transformed troponin I as a predictor of mortality in 2 independent populations.

BACKGROUND

The troponin I result is typically dichotomized by a single diagnostic cutoff. Its performance as a continuous prognostic variable has not previously been well-characterized.

METHODS

We studied the first troponin I sent from the emergency department (ED) as a predictor of all-cause inpatient mortality, with retrospectively gathered data. We performed our study in 2 stages, deriving our model with data from a single medical center and validating it with data from another. Subjects included every patient who had a troponin I sent from the ED during the period from November 2002 to January 2005. We assessed prognostic independence by including other potential confounders in nested logistic regression models. The troponin assay was identical at both sites (Ortho-Clinical Diagnostics, Rochester, New York).

RESULTS

There were a total of 34,227 patients (12,135 derivation and 22,092 validation). Odds ratio for mortality as a function of log10-troponin was 2.08 (95% confidence interval [CI] 1.85 to 2.32) in the derivation set and 2.07 (95% CI 1.92 to 2.24) for the validation set. Troponin I remained a strong predictor after inclusion of age, electrocardiogram normality, renal insufficiency, arrival mode, chief complaint, admission diagnosis, and abnormal vital signs into bivariate and nested multivariate models.

CONCLUSIONS

The presence of any detectible troponin I at ED presentation is associated with increased inpatient mortality. In 2 distinct clinical populations, the odds of death approximately doubled with any 10-fold increase in troponin result. This held true at levels below current diagnostic cutoffs. The placement and utility of dichotomous cutoffs might merit reconsideration.

Biomarkers in acute cardiac disease: the present and the future

The use of biomarkers to aid diagnosis and treatment is increasing rapidly as genomics and proteomics help us expand the number of markers we can use and as an improved understanding of the pathophysiology of cardiac disease guides their use. However, as with all rapidly expanding fields, there is the risk of excessive enthusiasm unless we are circumspect about the data that guide the clinical use of these new tools. This review focuses first on how to use troponin, which at present is the best validated of the new markers, and will hopefully provide insight into how to use this biomarker more productively by distinguishing subsets of patients and by providing an understanding of the meaning of elevations in various clinical situations. The review then discusses the use as well as the knowledge gaps associated with emerging biomarkers such as B-type natriuretic peptide and C-reactive protein, which are increasingly moving toward more productive clinical use. Finally, it reflects on some of the large number of markers that are still in development.

Non ST segment elevation acute coronary syndromes: A simplified risk-orientated algorithm

Non-ST segment elevation acute coronary syndromes (NSTE ACS) include a clinical spectrum that ranges from unstable angina to NSTE myocardial infarction. Management goals aim to prevent recurrent ACS and improve long-term outcomes by choosing a treatment strategy according to an estimate of the risk of an adverse outcome. Recent registry data suggest that patients with NSTE ACS frequently do not receive recommended treatment, and that risk stratification is not used to determine either the choice of treatment or the speed of access to coronary angiography. The present article evaluates the evidence for recommended treatment using information from recent trials and guidelines published by the major cardiac organizations in Europe and North America. Using this information, a multidisciplinary group developed a simplified algorithm that uses risk stratification to select an optimal early management strategy. Long-term outcomes are improved by a multi-faceted vascular protection strategy that is initiated at the time of hospitalization for NSTE ACS.

Prognostic factors in patients with minor troponin-I elevation but without acute myocardial infarction

OBJECTIVES

Although cardiac troponin I is widely used as a marker for myocardial infarction, its minor elevations are also observed in other clinical situations, and the prognostic factors in such clinical settings have not been well established. The aim of this study was to identify predictors of mortality in patients with minor troponin elevations without an acute myocardial infarction.

METHODS

We consecutively enrolled 134 patients from the emergency department with a peak troponin I level greater than the lower limit of detectability (0.04 ng/ml) but less than the 10% coefficient of variation cutoff value for diagnosis of myocardial infarction (0.26 ng/ml). These patients had chest pain or nonspecific symptoms of a circulatory abnormality but lacked the traditional features of an acute myocardial infarction. End point was defined as death from all causes. Cox regression analysis was used to test relations between clinical and biochemical variables and the outcome.

RESULTS

During the follow-up of 7.6+/-7.4 months, 12 patients died. Age, log creatine kinase myocardial isoform, and log C-reactive protein were found to be significantly correlated with death. After adjusting for possible confounders in the multivariate model, age (hazard ratio 1.09, confidence interval 1.02-1.16, P=0.012), log creatine kinase myocardial isoform (hazard ratio 13.11, confidence interval 2.01-85.52, P=0.007), and log C-reactive protein (hazard ratio 1.64, confidence interval 1.02-2.56, P=0.041) were identified as independent predictors of mortality.

CONCLUSIONS

Creatine kinase myocardial isoform and C-reactive protein levels and age can be integrated to risk-stratify patients with minor troponin I elevation for reasons other than acute myocardial infarction.

The diagnostic value of troponin T testing in the community setting

BACKGROUND

Many patients presenting with chest pain to their family physician are referred to the emergency room, in part, due to lack of accurate objective diagnostic tools. This study aimed to assess the diagnostic value of bedside troponin T kit testing in patients presenting with chest pain to their family physician.

DESIGN

Prospective, multi-center study.

METHODS

Consecutive subjects with chest pain were recruited from 44 community clinics in Jerusalem. Following clinical assessment by the family physician, qualitative troponin kit testing was performed. Patients with a negative clinical assessment and negative troponin kit were sent home and all others were referred to the emergency room. The final diagnosis at the time of hospital discharge was recorded and telephone follow up was performed after 60 days. Positive predictive value, negative predictive value, sensitivity and specificity of troponin kit for myocardial infarction diagnosis and of family physician for hospitalization, were assessed.

RESULTS

Of 392 patients enrolled, 349 (89%) were included in the final analysis. The prevalence of myocardial infarction was 1.7%. The positive and negative predictive values of the troponin kit for myocardial infarction diagnosis were 100% and 99.7%, respectively. The positive and negative predictive values of the family physician's assessment to predict hospitalization were 41.4% and 94.1%, respectively.

CONCLUSIONS

Troponin kit testing is an important tool to assist the family physician in the assessment of patients with chest pain in the community setting. Troponin kit testing may identify otherwise undiagnosed cases of myocardial infarctions, and reduce unnecessary referrals to the emergency room.

Use of biomarkers in the emergency department and chest pain unit

The use of biomarkers of cardiac injury in the emergency department (ED) and observation unit settings has several nuances that are different and, therefore, worthy of its own set of use guidelines. The markers that are used, however, are the same. The primary marker of choice continues to be cardiac troponin (Tn). Other markers that have been used because of the need in the ED for rapid triage have been myoglobin and fatty acid binding protein. In addition, some centers still prefer less sensitive and less specific markers such as creatine kinase myocardial band (CK-MB). More recently, a push has occurred to develop markers of ischemia, such as ischemia modified albumin (IMA),to determine which patients have ischemia, even in the absence of cardiac injury. As troponin assays become more sensitive and method for use becomes better understood, the use of these other markers are being relegated to lesser and lesser roles. Markers of ischemia are useful, but at present, despite some enthusiasm, are not ready for routine use. Before describing the recommendations for clinical use of biomarkers in the ED, a basic understanding of some of the science and measurement issues related to these analytes is helpful.

Risk Factors for Myocardial Injury during Off-Pump Coronary Artery Bypass Grafting

BACKGROUND

Although off-pump coronary artery bypass grafting (CABG) is now used worldwide for coronary revascularization, the pre- and intraoperative risk factors for myocardial injury associated with the surgical procedure remain to be elucidated. We performed a multivariate analysis to investigate factors that contribute to myocardial injury during off-pump CABG.

METHODS

The study population consisted of 22 patients who underwent off-pump CABG without apparent intraoperative complications. Blood samples were obtained before surgery and at 3 and 12 hours after the last anastomosis and serum Troponin T (cTnT) levels were measured to assess myocardial injury. Patient characteristics and factors related to preoperative cardiac function and the intraoperative process were analyzed to determine their correlation with serum cTnT levels, and the Spearman's correlation coefficient (r(s)) was computed.

RESULTS

Neither age, preoperative cardiac function, time required for anastomosis, the number of grafts, nor the total amount of bleeding were associated with serum cTnT levels. Serum cTnT at 3 and 12 hours after completed anastomosis correlated with the product of mean systolic blood pressure and mean heart rate (double product) during anastomosis. The r(s) values at 3 and 12 hours were 0.62 (P = .002) and 0.58 (P = .004), respectively. With respect to the serum cTnT level at 12 hours, creatinine clearance (Ccr) had a slight effect on the serum cTnT values.

CONCLUSIONS

High blood pressure and an increased heart rate during anastomosis are unfavorable factors for off-pump CABG. However, strict control of the blood pressure and heart rate makes it possible to subject even patients at high-risk to off-pump CABG from the viewpoints of myocardial injury.

National data elements for the clinical management of acute coronary syndromes

Patients with acute coronary syndromes represent a clinically diverse group and their care remains heterogeneous. These patients account for a significant burden of morbidity and mortality in Australia. Optimal patient outcomes depend on rapid diagnosis, accurate risk stratification and the effective implementation of proven therapies, as advocated by clinical guidelines. The challenge is in effectively applying evidence in clinical practice. Objectivity and standardised quantification of clinical practice are essential in understanding the evidence-practice gap. Observational registries are key to understanding the link between evidence-based medicine, clinical practice and patient outcome. Data elements for monitoring clinical management of patients with acute coronary syndromes have been adapted from internationally accepted definitions and incorporated into the National Health Data Dictionary, the national standard for health data definitions in Australia. Widespread use of these data elements will assist in the local development of "quality-of-care" initiatives and performance indicators, facilitate collaboration in cardiovascular outcomes research, and aid in the development of electronic data collection methods.

Survival among hospital in-patients with troponin T elevation below levels defining myocardial infarction

BACKGROUND

Cardiac troponin T (cTnT) has an accepted place in the management of patients presenting with suspected acute coronary syndrome (ACS). Uncertainty remains about the significance and interpretation of elevated cTnT below the cut-off levels defining myocardial infarction (0.1 microg/l).

AIM

To compare the mortality risks for elevation of cTnT in the ranges 0.01-0.029 microg/l, 0.03-0.099 microg/l and <0.01 microg/l.

DESIGN

Retrospective record study in three hospitals.

METHODS

All cTnT measurements with values in the range >0.01-0.099 microg/l analysed during January 2002 were extracted from clinical biochemistry laboratory databases. Following agreed exclusion criteria, 179 patients with cTnT in the range 0.01-0.099 microg/l and 60 patients <0.01 microg/l were selected at random from across the three sites. Six-month follow-up was completed by review of case notes and contact with the patients' GP.

RESULTS

There was a graded increase in mortality with increasing cTnT, although only achieving statistical significance for patients in the 0.03-0.099 microg/l range. The graded increase in relative risk with cTnT was weaker after adjustment for potential confounding factors

DISCUSSION

We found a trend for worse survival with increasing cTnT within the range 0.01-0.099 microg/l in unselected patient populations presenting with possible acute coronary syndrome. This suggests that the combined effects of assay imprecision and co-morbidity should be taken into account when interpreting borderline elevation of cTnT. The use of a cut-off based on current standards of assay precision should be used to define the sensitivity of cTnT as biochemical evidence of ischaemic cardiac damage and as an indicator of mortality risk. This level is likely to be between 0.03 and 0.1 microg/l.