Rule In, Rule Out
Are two measurements of troponin alone sufficient to rule in and rule out acute myocardial infaction?
There has been a flurry of articles discussing the utility of more sensitive troponin assays. Many of these report increases in situations that were not generally seen before -- following surgery, in persons doing certain athletic events, in renal disease and other pathologies. This may create difficulties (lower specificity) in the ED.
Here I address the question "What is THE current protocol for possible AMI?" This question has been asked since CK and the CK-MB became popular and it seems to have reappeared.
This puzzles me, as data have existed since the early troponin I with a cut-off of 0.2 ng/mL. Studies then indicated that two, sometimes three values of troponin alone at T0, T45 and T90 min were adequate to rule out an MI and in most cases rule it in. Half of the MI patients had an increased TnI at T0 and 95% were elevated within 2 hours.
Recently a number of papers have proposed much the same protocol. For example, Aldous et al, claimed that an "accleratated diagnostic protococal consisting of a TIMI risk score of 0, no new ECG changes, and negative troponin at 0 and 2 hours post presentation safely identifies patients at low risk of ACS, in whom discharge without further evaluation can be considered." The sensitivity of this protocol to rule out an MI was greater than 99%,
As far as ruling in an ACS, samples from 850 patients were studied by Collinson and others and found that the measurement of admission myoglobin [area under the curve (AUC) 0.76] and CK-MB (AUC 0.84) were diagnostically inferior and did not add to the diagnostic efficiency of cTnI (AUC 0.90-0.94) or cTnT (AUC 0.92) measurement on admission and 90 min.