basic metabolic panel

**Components and typical reference ranges (adult):** - Sodium (Na+): 135-145 mEq/L - Potassium (K+): 3.5-5.0 mEq/L - Chloride (Cl-): 98-106 mEq/L - Bicarbonate (HCO3-/total CO2): 22-28 mEq/L - Blood urea nitrogen (BUN): 7-20 mg/dL - Creatinine (Cr): 0.6-1.2 mg/dL (varies by sex and muscle mass) - Glucose (fasting): 70-100 mg/dL - Calcium (total): 8.5-10.5 mg/dL

**Derived values (high-yield):** - **Anion gap** = Na - (Cl + HCO3). Normal is about 8-12 mEq/L. An elevated gap suggests high anion gap metabolic acidosis (MUDPILES: Methanol, Uremia, DKA/alcoholic or starvation ketoacidosis, Propylene glycol/Paraldehyde, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates). A normal gap (hyperchloremic) acidosis is seen with diarrhea, RTA, and saline infusion. - **BUN:Cr ratio**: >20:1 suggests prerenal azotemia (hypovolemia, heart failure, GI bleed). About 10-15:1 is normal, and intrinsic renal disease (e.g., ATN) usually gives a ratio <15:1. - **eGFR** is calculated from creatinine (CKD-EPI), along with age and sex.

**Clinical interpretation pearls:** - **Hyponatremia**: Evaluate volume status and serum osmolality (SIADH, hypovolemia, heart failure, cirrhosis, psychogenic polydipsia). Correct slowly (≤8-10 mEq/L per 24 h) to avoid osmotic demyelination syndrome (central pontine myelinolysis). - **Hyperkalemia**: Causes include renal failure, acidosis, hemolysis (pseudohyperkalemia), rhabdomyolysis, and K-sparing drugs/ACE inhibitors. ECG changes are peaked T waves, then widened QRS, then sine wave. Treat with IV calcium gluconate (membrane stabilization), insulin plus glucose, albuterol, and then eliminate K (loop diuretics, binders, dialysis). - **Hypokalemia**: Causes include diuretics, vomiting, and diarrhea. ECG shows flattened T waves and U waves. It can cause arrhythmias and weakness. - **Metabolic alkalosis** (high HCO3): Vomiting, diuretics, hyperaldosteronism. Hypochloremic and often hypokalemic. - **Elevated creatinine**: Prerenal, intrinsic (ATN, AIN, GN), or postrenal causes. Creatinine rises later than GFR drops and is affected by muscle mass. - **Glucose**: Fasting ≥126 mg/dL on two occasions, random ≥200 with symptoms, or HbA1c ≥6.5% is diagnostic of diabetes. Marked hyperglycemia with a high anion gap points to DKA, and with very high osmolality it points to HHS. Hyperglycemia lowers measured Na by about 1.6-2.4 mEq/L per 100 mg/dL of glucose above 100. - **Calcium**: Correct for albumin (corrected Ca = measured Ca + 0.8 × [4.0 - albumin]). Hypercalcemia ("stones, bones, groans, psychiatric overtones") is most often caused by primary hyperparathyroidism in outpatients and by malignancy in inpatients. Hypocalcemia causes tetany, Chvostek and Trousseau signs, and a prolonged QT.

**BMP vs. CMP vs. renal panel:** - BMP: the 8 tests above (ionized Ca is not included). - CMP: BMP plus albumin, total protein, ALP, AST, ALT, and bilirubin (14 tests). - Renal function panel: BMP-type tests plus albumin and phosphate.

**Test-taking tips:** - The BMP is the first-line lab in a patient with altered mental status, dehydration, arrhythmia, or suspected acid-base disturbance. - Pseudohyponatremia occurs with severe hyperlipidemia or hyperproteinemia (with the older flame photometry/indirect ISE methods). - Sample hemolysis falsely elevates K. - Medication effects to know: ACE inhibitors/ARBs and K-sparing diuretics (↑K), loop and thiazide diuretics (↓K, ↓Na with thiazides), trimethoprim (↑K, ↑Cr via blocked tubular secretion without a true GFR change), and NSAIDs (↑Cr, ↑K).