Can you explain how Troponin level is used and how it relates to PT?
Troponin is an enzyme and found in striated muscle cells. Our cardiac physicians have found that Troponin I has a greater sensitivity and specificity in the diagnosis of MI. There are three isoforms: two are found in cardiac muscle, and one is found in skeletal muscle. Cell necrosis leads to the release of troponins into the circulation. TnC and Tnl are only expressed from cardiac muscle and are the only two that are tested during severe ischemia and infarction.
The normal range for troponin is less than 0.04 U/L. If a person is positive for the troponin enzyme, it takes eight hours to show up in the bloodstream or in the lab value. The peak time is 12 to 16 hours. If a doctor is trying to diagnose or rule out a myocardial infarction, a typical lab order that you'll see is for Troponin x 3 with a series of eight hours apart. The other thing that's critical to remember with troponins is that it takes a week to get reabsorbed into your system.
At Henry Ford, we have specific troponin guidelines. If it's less than 0.04, it's negative for myocardial damage, and there are no restrictions. If the range is between 0.05 to 0.2, that is considered indeterminate. We then require our therapists to review the chart to figure out why troponins were ordered in the first place. Check to see if it's a one-time order, look for any notes that may indicate that a cardiac component is not suspected for the troponin elevation. Typically, those are cardiology notes or other notes in the chart after the troponin becomes positive. Troponin levels of 0.2 or greater are positive for myocardial damage. If there's a new onset increase, we provide no treatment at that time. Once it's determined that the patient has had an MI, therapy can usually be initiated within 24 hours of the diagnosis, if the patient is hemodynamically stable. In other words, we can start seeing them 24 hours after the peak of troponin.
We are always going to look for the trends with troponins; a new peak indicates that there's further cardiac damage. There might be multiple peaks in any one patient's trends. We will also look at the EKG to see if there have been changes. Many times, if there was a non-ST elevated MI, the EKG might come out okay, but the troponin level is high. That's the only way to tell if the patient had damage. However, it is important to recognize that there are conditions in which troponin is excreted into the bloodstream, where it is not an acute myocardial infarction. Some of these conditions include:
- Acute PE (pulmonary hypertension)
- Acute Pericarditis
- Acute or Severe Heart Failure
- Sepsis and/or Shock
- Renal Failure
- False-Positive Troponin
- Stroke or subarachnoid hemorrhage
- Hypertensive Emergency
It is also important to note that cardiac surgery is going to release enzymes, because of damage to the cardiac muscle.