Source: TeraRecon

A secondary analysis of patients with calcium scores of 600 or greater confirmed that pretest probability for coronary artery disease (CAD) and coronary calcium scoring are important factors in the effectiveness of CT angiography (CTA) to exclude or confirm the presence of obstructive CAD, according to a study published online Jan. 17 in Journal of the American College of Cardiology.

The results showed that CTA should not be extended to patients with substantial coronary calcification, according to an accompanying editorial comment.

Patients with calcium scores of 600 or greater, along with other patient subgroups, were excluded from the initial CORE-64 (Coronary Artery Evaluation Using 64-Row Multidetector Computed Tomography Angiography) international study. Armin Arbab-Zadeh, MD, PhD, from the division of cardiology at Johns Hopkins University in Baltimore, analyzed these 80 patients to evaluate the diagnostic accuracy of CTA in this population.

“An assessment of pretest probability of CAD may help predict the value of CTA for excluding or confirming the presence of CAD… However, the utilization of a coronary calcium score threshold for deciding to perform CTA remains controversial,” wrote Arbab-Zadeh and colleagues. Some physicians bypass CTA among patients with severe coronary arterial calcification as the probability of obstructive CAD increases with the coronary calcium score.

The researchers used the area under the receiver-operating characteristic curve (AUC) for detecting 50 percent stenosis by CTA. Among the 291 patients with calcium scores 600 or greater, the AUC was 0.93. It decreased to 0.81 for patients with scores of less than 600. The AUC was 0.93 for the entire cohort.

Arbab-Zadeh and colleagues also analyzed data according to absent, mild, moderate and severe calcification and pretest probability. “Positive predictive values were very good (88 to 98 percent) and negative predictive values were poor (50 to 64 percent) for patients with high pretest probability/known CAD and any coronary calcification (calcium scores greater than 0).”

The analysis highlighted four results:

1. Including patients with severe calcification did not change the overall performance of CTA to detect stenosis of 50 percent or greater.
2. Diagnostic accuracy to detect obstructive CAD was reduced among patients with calcium scores of 600 or greater compared with patient scores of less than 600.
3. Pretest probability of CAD and coronary calcium score have a large impact in negative predictive values for detecting obstructive CAD.
4. CTA provides optimal performance to rule out obstructive CAD in patients with low to intermediate pretest probability of CAD and mild coronary calcification or in patients with a calcium score of zero.

Because predictive values of diagnostic exams hinge on disease prevalence, the authors cautioned against blanket statements about the predictive value of CTA, “as even a test with high sensitivity and specificity displays a remarkable variability for predictive values within a clinically applicable range of disease prevalence.”

They pointed out that CTA delivered an excellent positive predictive value among patients with calcium scores of 600 or more and a low negative predictive value in this group, primarily because of high disease prevalence.

Arbab-Zadeh et al emphasized positive and negative predictive values of 87 percent and 90 percent, respectively, among patients with intermediate pretest probability for obstructive CAD.

They concluded by acknowledging the lack of a calcium score threshold for CTA. The study provided an effective performance among patients with low to intermediate pretest probability who have low calcium scores and in those with any pretest probability and a calcium score of zero. However, its negative predictive value was highly variable and typically poor in patients with high pretest probability and known CAD with any calcification as well as in patients with intermediate pretest probability and more than mild calcification.

“[There] is a continuous shift from what one may consider an effective test to a less effective one, with uncertainty of test performance in the transition zone,” they wrote.

In an accompany editorial, Steven E. Nissen, MD, from the department of cardiovascular medicine at The Cleveland Clinic Foundation, highlighted the unfavorable performance of CTA among patients with calcium scores greater than 600. “Regardless of the pretest likelihood of disease, the negative predictive value was 0.50 for quantitative assessment of CAD and 0.63 for visual assessment.”

Nissen recommended that CTA not be applied as a diagnostic study among patients with substantial calcification. Physicians need to consider whether CTA provides data that would allow deferral of cardiac catheterization in patients with known coronary disease. False positive and false negative rates of 10 to 15 percent in this group are too high to recommend CTA as an alternative to cardiac catheterization or stress testing in high-probability patients, according to Nissen.

The editorialist also noted that CTA is less useful than stress testing in patients with an intermediate probability of CAD. The key data in this group are whether obstructions are ischemia-producing. “An abnormal CTA may require another imaging procedure in many patients to determine the physiologic impact of observed stenosis,” he wrote.

Nissen called for additional studies to analyze a wide spectrum of patients, including those who are not ideal candidates for CTA. “Pending such evaluation, coronary imaging using CTA should be used sparingly, with full recognition of the radiation burdens and risks of misdiagnosis.”