One reason the skeptical cardiologist has been so enthusiastic about coronary calcium (CAC) scans is that I have found them to be highly reproducible and highly accurate.
Unlike most imaging tests in cardiology if we perform a CAC on the same individual in the CT scanner of hospital A and then repeat it within a few days in the CT scanner of hospital B we expect the scores to be nearly identical.
Also, unlike most other imaging tests we don’t expect false negatives or false positives. If the CAC score is zero there is no coronary calcification-high sensitivity. If the score is nonzero there is definitively calcium and therefore atherosclerotic plaque in the coronaries-high specificity.
This is because calcium as defined in the Agatson score is literally black and white-a pixel is either above or below the cut-off. Computer software automatically identifies on the scan. A reasonably trained CT tech should be able to identify the calcium that is residing in the coronary arteries based on his or her knowledge of the coronary anatomy as registered on CT slices. Using software the total Agatson score is calculated.
A physician reader (either cardiologist or radiologist) (who should have a very good understanding of the cardiac and coronary anatomy ) should review the CT techs work and verify accuracy.
A recent case report, however, has demonstrated that the above assumptions are not always true.
Franz Messerli, a pre-eminent researcher in hypertension and a cardiologist describes in fascinating detail a false-positive CAC scan he underwent in 2013. He was told he had a score of 804 putting him in a high risk category consistent with extensive plaque formation.
After consulting with cardiologist friends and colleagues he decided to put himself on a statin and aspirin despite having an excellent lipid profile.
Messerli assumed that the CAC score was not a false positive (although later in his article he indicates he had questioned the reading) writing:
“although one can always quibble with ST segments or wall motion abnormalities, on the CAC the evidence is rock-hard, you actually with your own eyes can see the white calcium specks! ‘Individuals with very high Agatston scores (over 1000) have a 20% chance of suffering a myocardial infarction or cardiac death within a year’—although I did not quite classify, this patient information coming from esteemed Harvard cardiology colleagues3 was hardly reassuring.
(His reference 3 for the 20% risk of MI or cardiac death in a year for CAC score >1000 is suspect. It is a 2003 “patient page” on coronary calcium in Circulation which does not have a reference for that statistic.)
A more recent study found patients with extensive CAC (CAC≥1000) represent a unique, very high-risk phenotype with CVD mortality outcomes (0.80%/yr) commensurate with high-risk secondary prevention patients (0.77%/yr) from the FOURIER trial)
Six years after the diagnosis Messerli was at a Picasso exhibition, “leisurely ambling between his Blue and Pink Period “when he developed chest pain.
To further evaluate the chest pain he underwent a coronary CT angiogram and this demonstrated pristine and normal coronary arteries, totally devoid of calcium.
He did have a lot of mitral annular calcification (MAC). The CCTA images below show how close the MAC is to the left circumflex coronary artery (LCX).
The slice above shows how the MAC would appear on the CT scan designed to assess coronary calcium. It’s position is very close to that of the circumflex but an experienced reader/tech should have known this was not coronary calcification.
MAC is a very common finding on echocardiograms, especially in the elderly and it is likely that this error is not an isolated one.
Dr. Messerli writes
After relating these findings to the cardiologist who did the initial CAC, he indicated that most likely someone mistook mitral annular calcification as left circumflex calcium. This was hardly reassuring, since I specifically had asked that obvious question after receiving the initial CAC
Around the time I read Messerli’s case report I encountered a similar, albeit not as drastic case. A CAC scan showed a significant area of calcification near the left circumflex coronary artery which was scored as circumflex coronary calcification.
The pattern of this calcification is not consistent with the known path of the circumflex coronary in this case. When it was eliminated from the scoring the patient had a zero score. The difference between a nonzero score and a zero score is hugely significant but for patients with scores >100 such errors are less critical.
I have also encountered cases where extracardiac calcium mimics right coronary calcification.
There are some important take-home points from my and Dr. Messerli’s experience.
- False positive CAC scans do occur. We don’t know the frequency. If the scans are not overread by a competent cardiologist or radiologist with extensive experience in cardiac CT these mistakes will be more common
When I asked Dr. Messerli about this problem he responded
I am afraid you are correct in that CAC scores are generated by techs and radiologists and cardiologist simply sign the report without verifying the data. Little doubt that MAC is most often missed.
- “Had my CHD been diagnosed a decade earlier, guidelines might well have condemned me to taking beta-blockers for the reminder of my days.6 This, as Philip Roth taught us in ‘The Counterlife’, might have had rather unpleasant repercussions.7
Until recently I had never read anything by Philip Roth but when he died last year I read his Pulitzer Prize winning 1987 novel American Pastoral and liked it. Given this Roth reference involving beta-blockers I felt compelled to get my hands on “The Counterlife.” The book is a good read (much better IMHO than American Pastoral) and one of the main plot points relates to the side effects (see my post on feeling logy) a character suffers from a beta-blocker. Stimulated by a desire to be able to perform sexually if taken of the medication, the character undergoes coronary bypass surgery and dies.
2. “As stated by Mandrola and true in the present case, ‘given the (lucrative) downstream testing that often occurs when coronary calcium is found in asymptomatic people, the biggest winners from CAC screening may be the testers rather than the tested’.”
I feel the CAC in the right hands should not lead to (lucrative or inappropriate) downstream testing in the asymptomatic (see my discussion on this topic here.)