American Association for Clinical Chemistry
Better health through laboratory medicine
October 2009 Clinical Laboratory News: Diagnostics Profiles

CLN Banner Logo

October 2009: Volume 35, Number 10

D-dimer Role Evaluated in Pulmonary Embolism Work-Up

Duke University researchers have found that use of an automated, immunoturbidimetric D-dimer assay combined with a clinical risk algorithm supports implementation of the assay as a first-line test in evaluating patients with low or intermediate probability of acute pulmonary embolism (Am J Roentgenol 2009;193:425–430). The investigators stopped short of recommending D-dimer as an initial test in patients with high risk of PE, because of the small number of samples of that sub-population in their analysis.

The study was prompted by concerns about the potential over-use of pulmonary CT angiography (CTA) in patients with suspected PE, as well as prior research indicating that qualitative latex agglutination D-dimer tests were not sensitive enough. Other reports have concluded that up to 90% of pulmonary CTA findings in patients with suspected PE are normal. At the same time, studies have found that when D-dimer results are negative in patients with low clinical probability of having PE, it can be ruled out safely without the use of diagnostic imaging. However, some clinicians still are reluctant to implement D-dimer testing into standard practice.

The study involved 627 emergency department patients who presented with clinically suspected PE. All underwent an evaluation consisting of: development of a risk score using the revised Geneva score algorithm; immunoturbidimetric D-dimer testing; and pulmonary CTA. The researchers used a D-dimer negative predictive value cutoff of 1.2 mg/L based on their own analysis; prior studies have found that the immunoturbidimetric assay can produce results similar to rapid ELISA, the most sensitive D-dimer test.

Based on revised Geneva scores, 44.8% of subjects were classified as having low clinical risk of PE, 52.6% as intermediate, and 2.6% as high. Overall, 28 patients had pulmonary CTA-confirmed PE, including six in the low, 17 in the intermediate, and five in the high probability group. Sensitivity, NPV and specificity were 100%, 100% and 25% in the low risk group, 100%, 100%, and 33% in the intermediate group, and 80%, 80% and 37% in the high risk group. The mean D-dimer value in patients with PE was 8.46 mg/L. Interestingly, one patient with PE who also was in the high clinical probability group had a D-dimer value below the NPV cut-off. According to the researchers, this finding supports their contention that a three-tiered rather than two-tiered risk stratification system is better in the work-up of suspected PE, so that a potentially false-negative D-dimer result does not dissuade clinicians from pursuing further diagnostic studies in patients at highest risk.

New methyl-BEAMing Technique Detects Early Stage Colorectal Cancer

A multinational team lead by Johns Hopkins and Case Western Reserve University researchers has developed a novel method for detecting methylation-associated colorectal cancer that was four times more sensitive than serum-carcinoembryonic antigen (CEA) testing in their study population (Nat Biotechnol 2009; 27:858–863). In analysis of serum, the methyl-BEAMing (beds, emulsion, amplification and magnification) technique had a sensitivity of 52% in detecting earlier stage cancers confined to the wall of the colon, versus 14% with serum CEA. In addition, methyl-BEAMing analysis of fecal DNA detected 45% of adenomas and 41% of carcinomas. This compares with a 25% to 27% detection rate reported for immunochemical fecal occult blood tests with specificity of 93% to 95%.

The technique involved PCR amplification of individual DNA molecules covalently attached to magnetic beads in aqueous nanocompartments suspended in a continuous oil phase. The beads contained DNA sequences specific for exon1 of the vimentin gene, which is hypermethylated in an estimated 53% to 83% of colorectal cancer cases. After amplification, the beads were hybridized with fluorescent probes, sorted and analyzed using flow cytometry or next-generation sequencing. The method accurately detected methylated vimentin DNA in a mixture that contained only a single DNA molecule of methylated sequences mixed with 1,000 copies of unmethylated vimentin exon 1 sequences, for a detection of 0.1% of methylation target. Using a cutoff of one methylated vimentin fragment in 2 ml of plasma per sample, the overall sensitivity of methyl-BEAMing was 59% and specificity was 93%. In contrast, a previously optimized methylation-specific PCR failed to detect methylated molecules when the fraction of methylated fragments was less than 6.2%.

The researchers contend that methyl-BEAMing is an improvement over typical diagnostic tests based on DNA methylation, which use bisulfate to convert cytosine residues to uracils, thereby further degrading the DNA and making it difficult to implement in samples with only small amounts of DNA. The authors believe the technique could be especially valuable in detecting early-stage colorectal cancers, whereas CEA is predominately used to track disease recurrence. Detection rates could be improved with the addition of other methylated genes associated with colorectal cancer.

UACR Linked to Heart Failure Outcomes

New research indicates that the urinary albumin to creatinine ratio (UACR) is a powerful and independent predictor of prognosis in patients with chronic heart failure (Lancet 2009; 374:543–50). Increased albumin excretion is an established risk factor for mortality, cardiovascular events and other outcomes, but its prevalence and importance in heart failure has not been established.

The analysis involved 2,310 North American participants in the Candesartan in Heart Failure: Assessment of Reduction in Mortality and Morbidity (CHARM) trial for whom UACR values were available. CHARM enrolled more than 7,000 patients with New York Heart Association class II to IV heart failure divided into groups based on left ventricular ejection fraction levels and angiotensin-converting enzyme inhibitor therapy who were randomly assigned to take candesartan—an angiotensin II type-1 receptor blocker—or placebo. Primary endpoints were death from a cardiovascular cause or admission for worsening heart failure.

The study found that elevated UACR was associated with increased risk of composite outcome and death even after adjustment for other risk factors like renal function, diabetes, and HbA1c levels. The adjusted hazard ratio for the primary endpoints was 1.43 in subjects with microalbuminuria versus normal albuminuria, and 1.75 for those with macroalbuminuria versus normal albuminuria. The adjusted values for death were 1.62 in patients with microalbuminuria versus normal albuminuria and 1.76 for those with macroalbuminuria versus normal albuminuria. Candesartan therapy did not reduce or prevent the development of excessive excretion of urinary albumin.

The authors concluded that “because UCAR is a simple, readily available clinical test that is widely used in primary and secondary care, it might be of value in risk stratification of patients with heart failure.” They called for further assessment of UACR’s incremental prognostic information in relation to other emerging biomarkers, including natriuretic peptides.