Tag Archives: laboratory values

Lab Values From Intraosseous Blood

The intraosseous access device (IO) has been a lifesaver by providing vascular access in patients who are difficult IV sticks. In some cases, it is even difficult to draw blood in these patients by a direct venipuncture. So is it okay to send IO blood to the lab for analysis during a trauma resuscitation?

A study using 10 volunteers was published last year (imagine volunteering to have an IO needle placed)! All IO devices were inserted in the proximal humerus. Here is a summary of the results comparing IO and IV blood:

  • Hemoglobin / hematocrit – good correlation
  • White blood cell count – no correlation
  • Platelet count – no correlation
  • Sodium – no correlation but within 5% of IV value
  • Potassium – no correlation
  • Choloride – good correlation
  • Serum CO2 – no correlation
  • Calcium – no correlation but within 10% of IV value
  • Glucose – good correlation
  • BUN / Creatinine – good correlation

Bottom line: Intraosseous blood can be used if blood from arterial or venous puncture is not available. Discarding the first 2cc of marrow aspirated improves the accuracy of the lab results obtained. The important tests (hemoglobin/hematocrit, glucose) are reasonably accurate, as are Na, Cl, BUN, and creatinine. The use of IO blood for type and cross is not yet widely accepted by blood banks, but can be used until other blood is available. NOTE: your lab may try to refuse the specimen due to “other stuff” (marrow) in the specimen. Have them run it anyway!

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Reference: A new study of intraosseous blood for laboratory analysis. Arch Path Lab Med 134(9):1253-1260, 2010.

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.

Serial Hemoglobin To Monitor Chest Tube Output? Huh?

I’ve already written about the (f)utility of serially monitor hemoglobin (Hgb) or hematocrit (Hct) levels when managing solid organ injury nonoperatively. What about if you are concerned with bloody output from a chest tube drainage system? Could it be of any use there?

Seems like a reasonable idea, right? Wrong. As always, think it through and do the math! Here are the questions you need to ask yourself:

  • What is the Hgb or Hct of the fluid coming out of the chest tube? At worst, it will be the same as the patient, assuming that pure, whole blood is coming out. But this is seldom the case. The fluid is usually described as “serosanguinous”, which is not very exact, but tells you that it is thinner than blood. And if it looks more like Kool-Aid, the concentration is very low indeed.
  • What is the volume in the container? Most collection systems will collect a maximum of 1 to 1.5 L of juice.
  • How fast is it coming out? These things almost never fill right in front of your eyes. It’s a slow process, with less than a few hundred ccs per shift.

Here’s a few hundred ccs of thin drainage in a collection system. Probably decrease in Hgb value – < 0.1, which is far less than the range of lab error.

Bottom line: So now do the math. Let’s say the fluid has half the hematocrit / hemoglobin of whole blood. Losing one unit (500cc) of whole blood will generally drop your Hgb by about 1 gm, or your Hct by about 3%. If the blood is half-strength like I am proposing (and the usual drainage is typically much thinner), it will take twice as much (one liter) loss to drop the lab values by that much. This will probably come close to filling up the average collection system. If it takes a day or two or more to fill up, you are not going to see much change in their lab values. And most of the time, the blood in the system is thin like Kool-Aid, so your patient is really losing very little actual blood.

So measuring serial hemoglobin / hematocrit as you watch a hemothorax drain doesn’t make sense. Unless the output is pure blood and the system is filling up in front of your eyes, of course. In that case, a trip to the OR to fix the problem might be a better idea than doing a blood draw and sitting around waiting for the result to come back.

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