Body · lesson 9 of 11
CBC, liver & kidney — the routine safety net
The CMP and CBC in plain English: eGFR, ALT/AST, electrolytes, and red/white cells & platelets.
7 min read · reviewed October 2026
Almost every blood draw includes two workhorse panels by default: the CMP (comprehensive metabolic panel) and the CBC (complete blood count). They rarely get the attention that cholesterol or glucose do, but together they're a cheap, wide safety net — quietly screening your kidneys, liver, electrolytes, and blood cells.
This lesson translates the alphabet soup into plain English so the lines on your report stop being intimidating.
The CMP — kidneys, liver & electrolytes
Creatinine is a muscle waste product your kidneys clear; from it (plus age and sex) labs CALCULATE eGFR, an ESTIMATE of how well your kidneys filter. Higher eGFR is generally better. Because creatinine depends on muscle mass and hydration, a single value can be nudged by a hard workout or dehydration — the trend is what matters.
Blood urea nitrogen — another waste product the kidneys clear. Interpreted alongside creatinine; it's also influenced by hydration and protein intake, so it's read in context rather than alone.
Enzymes that leak into the blood when liver cells are stressed or damaged, so RAISED levels can flag liver issues (commonly fatty liver, alcohol, certain medications). AST also comes partly from muscle, so intense exercise can transiently raise it. A mildly elevated ALT is one of the most common — and most overlooked — findings, often pointing at metabolic/fatty liver.
Alkaline phosphatase (ALP) and bilirubin round out liver/bile-duct screening. Bilirubin is the pigment from old red cells; mild elevations are sometimes benign (e.g. Gilbert's syndrome). Patterns across these values help a clinician localize where a problem is.
Sodium, potassium, chloride, CO2 (bicarbonate), plus calcium, total protein/albumin, and fasting glucose. These reflect fluid balance, kidney handling, and overall chemistry. Potassium especially must stay in a tight range — both low and high are clinically important.
Why eGFR is an ESTIMATE, and creatinine is context-dependent
eGFR isn't measured directly — it's calculated from creatinine, age, and sex. Creatinine rises with more muscle mass, a recent hard workout, or dehydration, any of which can transiently lower a calculated eGFR without real kidney decline. So one borderline eGFR is usually repeated and read in context, not acted on alone.
How it works →
Creatinine is a waste product produced at a roughly steady rate from muscle, then filtered by the kidneys — so for a given person, blood creatinine inversely tracks filtration. The catch: production scales with muscle mass, so a very muscular person makes more creatinine and can show a 'lower' eGFR despite healthy kidneys, while someone with low muscle mass can have falsely reassuring numbers. Intense exercise transiently raises creatinine (muscle turnover), and dehydration concentrates it. eGFR equations adjust for age and sex but cannot fully account for body composition — which is why they're labeled estimates.
What the studies show →
The limitations of creatinine-based eGFR (muscle-mass dependence, exercise and hydration effects) are well recognized; cystatin C-based equations exist partly to address them. What's individualized is when a borderline eGFR warrants concern versus simple confirmation under standard conditions. The reliable practice is to repeat a borderline value rested and well-hydrated, watch the trend, and interpret it with a clinician rather than reacting to a single reading.
The CBC — your blood cells, counted
These reflect your oxygen-carrying capacity. LOW values point toward anemia (iron, B12, or folate deficiency; blood loss; chronic disease); HIGH values can reflect dehydration or other causes. Accompanying indices (like MCV, the average red-cell size) help classify the TYPE of anemia — small cells often mean iron deficiency, large cells often B12/folate.
Your immune cells. A high count often accompanies infection or inflammation; a low count can reflect certain infections, medications, or other conditions. The DIFFERENTIAL breaks WBCs into types (neutrophils, lymphocytes, etc.), which hints at what kind of challenge the body is facing.
Cell fragments essential for clotting. Too few raises bleeding risk; too many can occur with inflammation or other conditions. Like the rest of the CBC, a single mildly off value is usually repeated and interpreted in context rather than acted on immediately.
Why a mildly raised ALT is worth a second look
A mildly elevated ALT is one of the most common quiet findings on routine bloodwork — and it most often points at fatty liver tied to metabolic health, not heavy drinking. It's an easy line to skim past, but it can be an early flag worth acting on with diet, weight, and activity.
How it works →
ALT (alanine aminotransferase) is an enzyme concentrated in liver cells; when those cells are stressed or damaged, ALT leaks into the blood, so a rising level signals liver stress. The most common cause of a mildly high ALT today is metabolic-associated fatty liver, where excess fat accumulates in liver cells — closely linked to insulin resistance, central adiposity, and the same drivers as cardiometabolic disease. AST, a related enzyme, also comes partly from MUSCLE, so intense exercise can transiently raise it; the AST/ALT pattern helps a clinician distinguish causes (e.g. alcohol-related liver injury classically raises AST more than ALT).
What the studies show →
The link between mildly elevated ALT and fatty liver / metabolic dysfunction is well documented, and fatty liver is increasingly recognized as common and often reversible with weight loss and metabolic improvement. What's NOT a fixed personal threshold is the 'normal' ALT cutoff — proposed upper limits have been revised downward over time and vary by lab and sex. So treat a mildly high ALT as a clue to investigate (often metabolic), confirmed and interpreted with a clinician, rather than a diagnosis.
Your calculated eGFR comes back slightly below the reference range after a hard week of training and a long run that morning. What's the most reasonable first step?
- Assume permanent kidney damage and stop all exercise
- Recognize creatinine can rise (and eGFR fall) transiently with muscle mass, intense exercise, and dehydration — repeat the test rested and well-hydrated, and review the trend with a clinician
- Ignore it entirely; eGFR never matters
- Conclude the lab made an error
Show the answer →
B.Recognize creatinine can rise (and eGFR fall) transiently with muscle mass, intense exercise, and dehydration — repeat the test rested and well-hydrated, and review the trend with a clinician
eGFR is estimated from creatinine, which is influenced by muscle mass, recent intense exercise, and hydration. A single borderline value under those conditions is best confirmed by repeating it rested and hydrated, then interpreting the trend with a clinician — not treated as a standalone diagnosis.
Record eGFR, ALT/AST, electrolytes, and your blood-cell counts over time so a slow drift in your routine panels surfaces as a line — long before it would ever be felt.