Step by Step
Na/K
Sodium and potassium
Sodium (135–145 mEq/L): low sodium (hyponatremia — from SIADH, heart failure, or over-hydration) causes confusion and seizures; high sodium (hypernatremia — from dehydration or diabetes insipidus) causes thirst and dry mucous membranes. Potassium (3.5–5.0 mEq/L): low potassium causes muscle weakness and arrhythmias (with a U wave on ECG); high potassium causes peaked T waves and can progress to cardiac arrest.
Cr/BUN
Creatinine and BUN — kidney function
Creatinine (0.6–1.2 mg/dL) is a kidney waste product and the best simple kidney marker — elevated levels suggest kidney disease or decreased GFR. BUN (blood urea nitrogen, 7–20 mg/dL) reflects urea; a BUN:creatinine ratio above 20 suggests a prerenal cause.
Glu
Glucose
Fasting glucose (70–100 mg/dL): elevated levels suggest diabetes; low levels (hypoglycemia) cause tremor, diaphoresis, and confusion.
HCO₃
Bicarbonate
Bicarbonate (22–29 mEq/L) serves as the primary metabolic acid-base marker, reflecting the kidneys' role in regulating blood pH.
Applied Walkthrough
1
A patient presents with confusion and is found to have severely low sodium (hyponatremia) — this abnormality alone could explain the confusion, and the clinician investigates causes like SIADH, heart failure, or over-hydration.
2
A different patient's ECG shows peaked T waves, prompting a check of their potassium level — which comes back significantly elevated (hyperkalemia), a potentially life-threatening finding requiring urgent treatment before it progresses to cardiac arrest.
3
A third patient's creatinine is elevated, with a BUN:creatinine ratio above 20 — suggesting a prerenal cause (like dehydration or heart failure reducing kidney perfusion) rather than intrinsic kidney damage itself.
4
Meanwhile, a fourth patient's fasting glucose comes back well above 100 mg/dL on repeated testing — consistent with diabetes, prompting further workup and management.
Exam Application
Exams test whether you know the normal ranges for the seven core BMP values, whether you can interpret high versus low results for sodium, potassium, and glucose, and whether you can use the BUN:creatinine ratio to localize a kidney problem.
⚠ Common Trap
The most common trap is treating creatinine and BUN as interchangeable — while both reflect kidney function, the BUN:creatinine RATIO carries additional diagnostic information (distinguishing prerenal from intrinsic renal causes) that neither value alone provides.
✓ Quick Self-Check
1. What does the SUCKS BP mnemonic help you remember?
The seven core BMP values: Sodium, Urea, Creatinine, Potassium (K+), Sugar (glucose), Bicarbonate, pH.
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2. What are the symptoms of hyperkalemia, and what serious outcome can it cause?
Peaked T waves on ECG; it can progress to cardiac arrest.
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3. What is considered the best simple marker of kidney function?
Creatinine.
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4. What does a BUN:creatinine ratio above 20 suggest?
A prerenal cause (like dehydration, GI bleeding, or heart failure) rather than intrinsic kidney damage.
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5. What symptoms does hypoglycemia cause?
Tremor, diaphoresis, and confusion.
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