Video 6: Understanding Lab and Imaging Tests

Dr. Nelson (00:00):

Welcome back, student doctors! I’m Dr. Nelson. It’s a pleasure being your colleague. As we take care of virtual patients in our Full Code emergency department, let’s turn our next discussion towards investigation via labs and imaging and other testing. 

Whenever we perform any lab or imaging investigation, we always ask ourselves, what is this lab or imaging test going to do for our decision making? We think about whether the test will be positive or negative, and how will that impact what we do going forward. 

For example, if we order a lab test, whether it’s positive or negative, we’re going to do the same thing moving forward. Why order it? It’s just a waste of resources at that point. We really try to stay diligent of what will help us in making our decision as far as diagnosis or as far as treatment. And will this lab or imaging tests point us in a different direction, based on whether it’s positive or negative?

Dr. Nelson (01:03):

Another thing that we think about when we’re ordering these imaging and lab tests are their inherent flaws. Each lab and imaging test has what we call sensitivity and specificity. Now, those are pretty complicated terms, but I’m going to try to simplify it a little bit for you. 

Sensitivity means what are the odds that if you have a positive test, that you really have the actual disease? The specificity is kind of the opposite of that. It says, what are the odds if you have a negative test, that you truly don’t have that condition? That really adds multiple layers to our decisions in choosing which labs and imaging that we are going to perform. 

For example, let me illustrate. We might order a complete blood count for a patient looking at the different cells in our blood, red cells, white cells, platelets, and the differential of them, whether they’re neutrophils, lymphocytes, and so forth amongst our white cells.

Dr. Nelson (02:03):

And let’s say we’re trying to rule out a condition like meningitis. Having an elevated white blood cell count doesn’t always mean that you have meningitis, nor does having a low white blood cell count mean you don’t have meningitis. Now, in our patient case who is hypoventilating is hypoxic and is difficult to awaken, we might order things like an arterial blood gas and that will check the level of oxygen and carbon dioxide in his blood. We would order a chemistry level to determine things like his sodium level, potassium, his kidney functions. We might check out a blood sugar too to see [if this is] some sort of issue with his glucose causing him to not function well. We could order things like an ultrasound. We do something called POCUS, or point-of-care ultrasound, where we can use an ultrasound as kind of a stethoscope, getting a to guide our decision making.

Dr. Nelson (03:00):

We can order an electrocardiogram, which is a more formal investigation of the wiring of the heart. We could order x-rays, CT scans, all kinds of different labs. In this case, I would order also a complete blood count to see if there’s some sort of infectious problem at the root of our condition and a toxicology screen to see if there’s been some potential either accidental or intentional overdose. So this brings us back to our AAMC Competencies, and if you review those, it would be relevant for competencies like adaptability, scientific inquiry, or critical thinking and analysis. 

Now, I want you to stop this video again and talk amongst your group about what you would potentially order in our patient case and what implications are there in ordering labs and imaging and other tests.

• What would you order in our patient case?
  
  
• What implications are there in ordering labs and imaging and other tests?
  
  Here are a few extra resources to help you answer the second question:
  
  
  • Yeshoua et al. (2023)
  • Zare et al. (2021)