American Association for Clinical Chemistry
Better health through laboratory medicine
April 2007 Clinical Laboratory News: Diagnostic Profiles

April 2007: Volume 33, Number 4

Labs Should Evaluate “Panic” Limits for Sodium
Serum and whole blood sodium critical values that indicate imminent danger to patients should be ≤120 mEq/L and ≥155 mEq/L, researchers recommended in a new study published in the American Journal of Clinical Pathology (2007; 127: 56–59). The team from State University of New York Downstate studied all critical serum and whole blood sodium results called in to clinicians during a 6-month period and reviewed endangered patients’ electronic medical records for clinical responses and patient outcomes. Of 111,545 sodium results reported during the study, 615 or 0.6% were critical. Using criteria of ≤120 mEq/L and ≥155 mEq/L, researchers found 166 critically low results and 447 critically high results. In these hypernatremic and hyponatremic patients, hospital lengths of stay were longer than the institution’s average and the mortality rates were 19% and 48%, respectively. The researchers also measured how long it took for clinicians to respond to these critical values and found that they acted on the critical values of more than 50% of patients within 4 hours. “Because serious clinical manifestations do not usually occur until serum sodium results are in the 158 to 160 mEq/L range, 155 mEq/L is a reasonable upper critical value limit,” researchers wrote. Recognizing that changing the critical limit for sodium would increase workload dramatically for many labs and clinicians, the authors suggested that labs that use 160 mEq/L as the upper critical limit should evaluate their patient populations before lowering it.
CRP Predicts COPD Outcomes 
An increased serum CRP level is a strong, independent predictor of future chronic obstructive pulmonary disease (COPD) outcomes in patients with airway obstruction, according to a newly published study in the American Journal of Respiratory and Critical Care Medicine (2007; 17: 250–255). Danish researchers assessed the relationship between CRP and systemic inflation by measuring CRP in 1,302 individuals at baseline and by recording COPD admissions and deaths as outcomes during 8 years of follow-up. During that time, 185 (14%) individuals were hospitalized due to COPD and 83 (6%) died of COPD. Those individuals with baseline CRP of >3 mg/L had higher rates of hospitalization and mortality rates due to COPD than counterparts with lower CRP levels. After adjusting data for sex, age, predicted forced expiratory volume in 1 second (FEV1%), tobacco consumption, and ischemic heart disease, researchers found the hazard ratios for hospitalization and death due to COPD were 1.4 and 2.2 (95% CI, 1.0–2.0 and 1.2–3.9, respectively) in individuals with baseline CRP of >3 mg/L, versus patients with levels ≤3 mg/L. The absolute 10-year risks for COPD hospitalization and death in individuals with CRP >3 mg/L were 54% and 57%, respectively, among patients older than 70 years who consumed more than 15g/day of tobacco and had an FEV1% predicted of less than 50. These findings imply that clinicians can calculate 10-year mortality risk for COPD and use this information to counsel patients, researchers wrote.
New ACS Risk Factor Identified
Low levels of blood eicosapentaenoic acid (EPA) plus docosahexaenoic acid (DHA) in blood omega-3 (ω-3) fatty acids (FAs) may be an independent and modifiable risk factor for acute coronary syndromes (ACS), at least in middle aged patients, but higher blood trans FA content is not, according to an article published in American Journal of Cardiology (2007; 99: 154–158). Researchers analyzed the ω-3 and FA composition of whole blood from 94 patients with ACS in two Kansas City, Mo. hospitals and 94 age-, gender-, and race-matched controls using multivariable models. They adjusted results according to smoking status, alcohol use, diabetes, body mass index, serum lipids, and history of myocardial infarction or revascularization. The subjects’ mean age was 47 years, 54% were men, and 80% were Caucasian. Whole blood long-chain ω-3 FA plus DHA content was 29% lower in the ACS patients than in the controls (1.7 ± 0.9% vs 2.4 ± 1.4%), but trans FA content in the two groups was not much different (2.1 ± 0.7% vs 2.0 ± 0.9%,). The multivariable-adjusted odds for case status was 0.67 (95% CI, 0.46-0.98) for a 1 SD increase in blood EPA + DHA. The inclusion of trans FAs in the EPA + DHA model did not alter this association. The researchers noted several factors associated with lower ω-3 levels, including smoking, no college education, male gender, low HDL cholesterol levels, and high triglyceride levels.
Troponin Alone Doesn’t Diagnose MI
Although cardiac troponin is a sensitive and specific biochemical marker of myocardial damage, clinicians should rely primarily on clinical presentation to diagnose a myocardial infarction, according to research published in Archives of Internal Medicine (2007; 167: 276–281). Noting the challenge of determining if troponin elevation is actually the result of a coronary event, Israeli researchers concluded that age, renal function, and maximal troponin value can help improve the accuracy of diagnosis of acute coronary syndrome. They proposed an algorithm that considers these factors—along with clinical presentation and electrocardographic changes—based on data collected at baseline and during 2.5 years of follow-up from 615 patients who initially presented with troponin T elevation at two hospitals of Hadassah-Hebrew University Medical Center in Jerusalem during 2003. Researchers divided patients into groups with ACS and nonthrombotic troponin elevation and collected demographic, clinical, and mortality data. After performing logistic regression and survival analysis, they calculated positive predictive value for diagnosis. Fifty-three percent of patients had a main diagnosis of ACS, 41% had nonthrombotic tronponin elevation, and 6% had no conclusive diagnosis. The researchers identified several positive predictors of ACS diagnosis: 40–70 years of age; a history of hypertension or ischemic heart disease; normal renal function; and a troponin T level >1.0 ng/mL. By itself, troponin T had a positive predictive value of only 56% (95% CI, 52%-60%), and in patients with levels of ≤1.0 ng/mL, its predictive value was as low as 48%. That’s not enough to rule in MI, researchers maintained. In patients with troponin T levels >1.0 ng/mL and normal renal function, the likelihood of having ACS was 90%, regardless of age, but that likelihood was as low as 27% for elderly renal failure patients with values of 0.1 to 1.0 ng/mL. In-hospital and long-term survival rates were significantly better for patients with ACS.