🧪 Lab Values & Diagnostics Lesson

BMP: the "fishbone" panel ordered every day in hospitals

Seven values that together give a snapshot of kidney function, electrolyte balance, and blood sugar — one of the most frequently ordered panels in all of medicine.

136-145

3.5-5.0

98-106

CO2

22-29

BUN

7-25

0.6-1.2

Gluc

70-100

📖 Full Breakdown

Seven values, and the two ratios that unlock the most clinical meaning

Individually these are just numbers — but two specific ratios (BUN/Cr and Na patterns) turn them into diagnostic tools.

Sodium (Na+)

136–145 mEq/L

Low sodium (hyponatremia) can cause confusion and seizures. High sodium (hypernatremia) typically reflects dehydration.

Potassium (K+)

3.5–5.0 mEq/L

A narrow, dangerous range — values outside it risk cardiac arrhythmias, making potassium one of the most closely monitored electrolytes in acute care.

Chloride & Bicarbonate (CO2)

98-106 and 22-29 mEq/L

Bicarbonate reflects the metabolic acid-base status; low bicarbonate suggests acidosis.

BUN and Creatinine

7-25 mg/dL and 0.6-1.2 mg/dL

Both rise with kidney dysfunction, but the RATIO between them is the real diagnostic tool: greater than 20:1 suggests a pre-renal cause (like dehydration), while less than 10:1 suggests intrinsic kidney disease.

Glucose

70–100 mg/dL fasting

Values above 126 mg/dL on two separate occasions define diabetes; values below 70 indicate hypoglycemia.

🩺 Clinical / Exam Application

A dehydrated patient's labs show BUN elevated out of proportion to creatinine, with a BUN:Cr ratio over 20:1. Because the kidneys themselves aren't damaged in simple dehydration — they're just receiving less blood flow — creatinine (a marker purely of kidney filtering capacity) stays relatively normal while BUN (also affected by hydration status) rises more. This ratio lets a clinician distinguish "kidneys not getting enough blood" from "kidneys are actually damaged" using two numbers from the same routine panel.

⚠️ Exam Alert

The BUN:Cr ratio is one of the most frequently tested calculations from the BMP — memorize that greater than 20:1 suggests pre-renal azotemia (like dehydration or heart failure), while a ratio closer to 10:1 or lower suggests intrinsic renal disease.

🚧 Common Trap

Don't assume an elevated BUN alone means kidney damage. BUN is also affected by dehydration, high protein intake, and GI bleeding — it must be interpreted alongside creatinine and the BUN:Cr ratio, not in isolation.

✅ Quick Check

A patient has a BUN:Cr ratio of 25:1. Does this suggest pre-renal (like dehydration) or intrinsic kidney disease?

📝 Exam Prep

Common Exam Questions

❓ What is the BMP and what conditions does it help diagnose?

✅ The BMP includes sodium, potassium, chloride, CO2, BUN, creatinine, and glucose. It is used to assess kidney function (via BUN/Cr), electrolyte balance, acid-base status, and blood glucose control.

❓ What does a BUN:Cr ratio greater than 20:1 suggest?

✅ It suggests a pre-renal cause of kidney dysfunction, such as dehydration or reduced blood flow to the kidneys, rather than intrinsic kidney damage.