Lab Series: What does a Comprehensive Metabolic Profile (CMP) tell us?
Quick Summary: Variations of a comprehensive metabolic profile (CMP) are performed in both urgent and routine medical evaluations. This panel gives us a window into your electrolyte balance and kidney function, blood sugar regulation, and liver health. Functionally, this lab can also tell us about your nutritional status - like protein intake.
The Big Picture.
A comprehensive metabolic profile (CMP) gives us a broad snapshot of critical aspects of your metabolism. While each of these metabolites can tell us about numerous functions in your body, we generally break the CMP up like this:
Kidney Function: BUN, Creatinine, eGFR, BUN/Creatinine Ratio, Sodium, Potassium, Chloride, CO2, and Calcium.
Acid/Base Balance: CO2
Blood Sugar: Glucose
Liver/Digestive Health: BUN, Albumin, Globulin, A/G Ratio, Bilirubin, Alkaline Phosphatase, AST, and ALT.
Nutritional Health: Total protein and electrolytes listed under kidney function.
The job of your kidneys (grossly oversimplified).
You might be shocked to discover that the job of your kidneys is not to make urine. While urine is a byproduct of kidney function, the ultimate goal of the kidneys is to continually filter and adjust the contents of our blood to be "just so."
Our kidneys regulate electrolyte balance to regulate our blood pressure and ensure that our heart still beats and our nerves can still fire. Our kidneys are also a significant regulator of acid-base balance (e.g., keeping our body's pH at 7.4). While some pH regulation happens at the bone and respiratory system level, the kidneys are the "long-term pH regulators." We use CO2 as a proxy for "acidity" in the body, and if this level is markedly abnormal, then we need to explore kidney function further.
Blood sugar is not well-screened for in a CMP.
The only way we look at blood sugar regulation in the CMP is by evaluating your fasting blood glucose. The problem is that, often, patients will have accidentally consumed calories within the 1-2 hours before their blood draw (ex: many folks need to remember that creamer/sweetener added to coffee or tea will break your fast).
When we see abnormally high or low blood glucose, we will (should) follow up with a fasting insulin and hemoglobin A1c (HbA1c) test. Other reasonable tests include a C-peptide and fructosamine.
Your liver, digestion, and nutritional status.
The liver is an amazingly complex and essential organ. One of the ways that we'll screen for liver health before busting out fancy specialized tests is to look at liver enzymes: AST and ALT.
These enzymes can be mildly elevated after heavy exercise or taking certain medications (e.g., statins, aspirin, and some botanical medicines). We typically do not worry about isolated, mild elevations. We explore beyond the CMP when the AST and ALT levels increase >10X normal.
Note: Many people refer to AST/ALT as "liver function tests," however, they don't give a proper window into "liver function." These enzymes increase in the blood when liver cells (i.e., hepatocytes) break down and release their contents into the blood - including these enzymes. More appropriately, AST/ALT gives us a window into liver dysfunction.
We assess both nutritional status and liver function when evaluating protein levels. For normal protein levels (e.g., BUN, Albumin, Globulin, Total Protein, A/G Ratio) to occur, we need an adequate intake from our diet and sufficient liver production.
Wrap Up.
As you can see, the CMP is not a stand-alone diagnostic panel. Even more than the CBC discussed last week, the CMP is a screening panel that works as a launching point for further physical examination, blood tests, or imaging studies. In the CBC and CMP, "normal values" are reliable indicators of "normal health."
In my practice, we work together to make you make you resilient before or after injuries. I offer in-person care here in Arizona and virtual concussion/PCS programs for folks outside the state and abroad. Over the course of three visits, we dive deeply into autonomic, metabolic, visual/vestibular, cervical, and psychological assessment and screening. By completing this comprehensive assessment in a matter of 2-3 weeks, patients are provided with a comprehensive rehab plan that can shave months off their recovery times.
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