Tag Archives: diagnostic test

Another Worthless Test? Serum / Urine Myoglobin

We often rely on diagnostic testing to augment our physical examination skills. These tests may be in the form of imaging that allows us to see things that we normally cannot, or measurements of body composition using laboratory testing.

If you look at the “menu” of tests that your hospital laboratory offers, it is very extensive. You can order just about any assay imaginable on any body fluid or tissue. Diagnosis of many of the clinical diseases or disorders that we treat has come to rely on some of these assays.

Let’s take rhabdomyolysis, for example. I’ve been writing about compartment syndrome in the last few posts. One of the byproducts of a full-blown compartment syndrome is muscle breakdown. Two of the well-known substances released from injured muscle are creatine kinase (CK) and myoglobin.

Many textbooks advise the clinician to monitor levels of these substances, since myoglobin is toxic to nephrons and may lead to kidney injury. So most trauma professionals routinely write orders for serial CK, myoglobin, as well as creatinine to monitor renal function.

But most clinicians do not know their laboratory as well as they think. Your lab has the capability to perform commonly requested tests rapidly and on site. But what about assays that are rarely ordered? Does it make sense to have the reagents necessary for these uncommon tests on hand at all times? They degrade over time, and may very well expire before they are ever used, costing money to replace.

So most hospital labs send uncommon assays out to larger labs that perform the test for a large geographic area. But how does the “send out” specimen get to that lab? By courier (if local) or more commonly, by delivery service (if remote). And obviously, this takes time. And some assays are complex and may take hours or days to perform.

At my hospital lab, a serum or urine myoglobin assay is a “send out” test. And if you ask, the lab will tell you that it takes 3-4 business days to get the result. So if you send it out Wednesday, you will have the result the following Monday! Does it make any sense to get serial myoglobins? Or even a single myoglobin test? By the time you get the result, your patient will be treated and gone!

Bottom line: Think about the tests that you order very carefully. If you are ordering something out of the ordinary, check with your lab. Is it a “send out” test? How long does it take to get a result? And more importantly, how expensive is it? These tend to be pricey due to the low frequency of processing.

Then do the math. Is it worth the wait and expense? Or can you get the same information using routine, in house testing? In the case of rhabdomyolysis, serum CK levels are good markers, as is visual inspection of the urine. If it’s any darker than a light yellow, there may be myoglobin present. A quick and dirty way to confirm is some inexpensive testing: a urine specimen that is dipstick positive for blood, and with microscopic analysis shows few if any RBCs usually means myoglobin. Voila! Diagnosis now, not in 4 days.

Another Failure Of Shotgun Style Diagnostic Testing: The Trauma Incidentaloma

When our patients present with a problem, there is a time honored and well-defined sequence to help us come to a final diagnosis. 

  • Take a detailed history
  • Examine the patient
  • Order pertinent diagnostic tests, if indicated
  • Then think about it a while

The first two items are a chip shot, and the trauma professional can gain a lot of information by spending a relatively short period of time doing these. And many times the diagnosis can be made without any further action.

However, diagnostic testing of all kinds has become so prevalent and easy to obtain that we rely on it a bit too much. And sometimes, we order it up in lieu of a thorough history and exam. If the clinician skimps on those steps, it’s much more difficult to narrow the list of differential diagnoses to a manageable number.

So what happens then? They use diagnostic tests as a crutch. Instead of being able to select a few focused tests to answer the questions, they essentially put an order sheet on the wall, fire off a shotgun, and order everything that’s been hit by the pellets. 

Lots of tests, so they will definitely find the answer, right? Nope! There are two major problems here. First, the so-called signal to noise ratio is very low. There are so many results, that it is easy to overlook a pertinent positive among all the negatives.

But more significantly, there is always the possibility that there will be more than one positive. One of them might actually be the answer you were seeking. But what about the others? There are the trauma incidentalomas. Some may be truly positive, but there is always the possibility of a false positive. These are the most treacherous, because many trauma professionals then feel obligated to “do something about it.” 

As we have found from multiple screening tests like PSA, PAP smear, and mammography, a significant number of patients may be harmed trying to further investigate what turns out to be nothing at all (artifact), or something completely benign. This includes not only harm from complications or unnecessary procedures, but months of anxiety the patient may suffer while the clinicians figure out what that thing inside them really is.

There are only a few studies on trauma incidentalomas available. One reviewed a series of almost 600 head CT scans in patients with TBI and found unexpected findings on 85%. About 90% were obviously benign. Unfortunately, it was not possible to follow these patients to find out how many of the remaining lesions turned out to be benign as well. But I would wager that most did.

Bottom line: I shouldn’t even have to say this, but do a good history and physical exam! If you need diagnostic studies, order only the one(s) that have the potential to make your final diagnosis. Don’t shotgun it. One very helpful tool is a well-designed practice guideline for commonly encountered clinical scenarios. This will limit the number of “other” findings you have to deal with. And finally, did I say to do a good history and physical exam?

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Reference: Incidental cranial CT findings in head injury patients in a Nigerian tertiary hospital. J Emerg Trauma Shock 8(2):77-82, 2015.