8.2 Laboratory Values: Metabolic Panel (BMP)

Basic Metabolic Panel: Interpret the Patient and the Trend

A basic metabolic panel (BMP) measures eight substances: glucose, calcium, sodium, potassium, chloride, carbon dioxide (reported as total CO₂/bicarbonate), blood urea nitrogen (BUN), and creatinine. Fasting may be requested for some tests or clinical purposes, but a BMP is not always a fasting test. Laboratories may use different reference and critical-result ranges. Compare each value with the patient's symptoms, baseline, medications, volume status, and the laboratory's reporting policy.

Sodium and water balance

An often-used adult sodium reference interval is approximately 135–145 mEq/L, but the meaning of a result depends on onset, cause, symptoms, and fluid status. Hyponatremia can be asymptomatic or cause headache, nausea, confusion, seizures, or coma; a value alone does not dictate that a particular symptom must be present. New confusion, seizure, or reduced consciousness warrants prompt assessment and escalation.

For severe symptomatic hyponatremia, guideline-directed treatment may use prompt, monitored 3% saline boluses to address cerebral edema, followed by careful reassessment of sodium and symptoms. It is wrong to say hypertonic saline must always be infused slowly. The team also limits later overcorrection, especially when hyponatremia may be chronic. Less urgent, cause-specific treatment may include fluid restriction for selected dilutional causes, but not for every patient with low sodium.

Hypernatremia often reflects a water deficit, but the volume state matters. In a patient with shock or severe hypovolemia, restore circulation first with an appropriate isotonic resuscitation fluid as ordered; subsequent free-water replacement is tailored and monitored. Do not prescribe a hypotonic fluid as the only first step for every patient with high sodium.

Potassium and the ECG

An often-used potassium reference interval is about 3.5–5.0 mEq/L. Hypokalemia can produce weakness, T-wave flattening, or U waves, but U waves are not pathognomonic. Hyperkalemia can produce peaked T waves, PR prolongation, loss of P waves, or QRS widening; presentations vary, and it can also mimic an ST-elevation pattern. A normal ECG does not exclude clinically important hyperkalemia. Interpret ECG and potassium together with renal function, symptoms, medication exposure, rate of change, and whether the specimen may be hemolyzed.

Potassium chloride concentrate is never given IV push. For IV replacement, use a diluted preparation, controlled infusion device, prescribed rate/route, product labeling, and the required laboratory and ECG monitoring. There is no universal “maximum 10–20 mEq/hour” rule for every patient and product; exceptionally severe hypokalemia may require a different closely monitored protocol. If hyperkalemia is suspected, review potassium sources and prescribed medications, obtain appropriate cardiac monitoring, and follow the ordered treatment plan. IV calcium protects the myocardium when indicated but does not lower potassium; insulin/glucose or an inhaled beta-agonist shifts potassium into cells; removal may require diuresis, a binder in selected circumstances, or dialysis. Bicarbonate is considered mainly in selected metabolic acidosis, not routine hyperkalemia treatment. Binders are not immediate rescue therapy for an unstable patient.

Creatinine, eGFR, BUN, and urine output

Creatinine is a muscle-metabolism product cleared by the kidneys. A rising value can signal reduced filtration, but a single high result does not diagnose AKI or CKD, and a normal value can miss early disease. Review the person's baseline trend, estimated GFR, muscle mass, diet, hydration, and other clinical data. A low creatinine may reflect low muscle mass, malnutrition, or serious liver disease rather than being automatically insignificant. Urine output should be interpreted against body size, duration, and the clinical situation; a fixed 30 mL/hour is not a universal AKI criterion.

BUN can increase with reduced kidney clearance, dehydration, high protein breakdown, or gastrointestinal bleeding. A BUN-to-creatinine ratio can be a clue, not a diagnosis: it is affected by hydration, diet, catabolism, bleeding, and both measurements' limitations.

Contrast and metformin: Do not teach a blanket rule to hold metformin for every contrast study. Current product labeling calls for stopping it at or before iodinated contrast in specified higher-risk circumstances (for example eGFR 30–60 mL/min/1.73 m², certain hepatic/alcohol/heart-failure histories, or intra-arterial iodinated contrast), then rechecking renal function 48 hours later before restart if stable. Confirm the product order and protocol for the actual patient.

Glucose and safe treatment

Glucose interpretation depends on fasting status and the patient's diabetes treatment. Hypoglycemia may cause tremor, sweating, hunger, confusion, seizure, or loss of consciousness, but not every patient becomes cold and clammy. If the person is alert and can swallow safely, use fast-acting oral carbohydrate under the care plan and recheck glucose. If the person cannot swallow safely, do not give oral food or drink. Severe hypoglycemia requires urgent assistance and emergency treatment such as prescribed glucagon or IV dextrose per protocol.

Hyperglycemia can cause thirst and polyuria; dry skin, fruity breath, or Kussmaul respirations may occur in specific dehydrating or ketoacidotic states, not in every high-glucose result. Treat the cause and assess fluids, electrolytes, and mental status as indicated. Laboratories set critical glucose call thresholds; do not treat one listed number as a universal critical value.

Exam-focused priorities

• A dangerous symptom or rhythm change can be more urgent than a modestly abnormal value in a stable patient.
• Check the laboratory's range and call policy, the trend, and whether the specimen may be unreliable.
• Never push IV potassium, and never give oral carbohydrate to someone who cannot swallow safely.
• Use ordered protocols and reassess after treatment; a single BMP result is a starting point, not a complete diagnosis.

Primary references: MedlinePlus BMP; MedlinePlus creatinine; European hyponatremia guideline; Emergency Care BC hypernatremia guidance; KDIGO acute hyperkalemia report; DailyMed potassium chloride label; DailyMed metformin label; ADA severe hypoglycemia.