Clinical Chemistry and Metabolic Analysis

The basic metabolic panel (BMP) is a set of seven or eight tests that assess electrolyte balance, kidney function, and glucose metabolism. Components: (1) Sodium (Na, mEq/L): maintains fluid balance and nerve function. Low = hyponatremia (fluid overload, SIADH, diuretics); high = hypernatremia (dehydration). Normal ~135–145. (2) Potassium (K, mEq/L): critical for heart rhythm and muscle. Low = hypokalemia (diuretics, diarrhea, alkalosis) — risk of arrhythmia; high = hyperkalemia (kidney disease, acidosis) — risk of cardiac arrest. Normal ~3.5–5.0. Potassium is the most critical electrolyte to monitor because abnormalities cause dangerous arrhythmias. (3) Chloride (Cl, mEq/L): maintains acid-base balance and fluid balance. Low = hypochloremia (vomiting, diuretics, loop diuretics); high = hyperchloremia (diarrhea, kidney disease). Normal ~98–107. (4) CO2 (bicarbonate, mEq/L): buffers blood pH. Low = metabolic acidosis (diabetes, kidney disease, diarrhea); high = metabolic alkalosis (vomiting, diuretics). Normal ~23–29. (5) Glucose (mg/dL, fasting): energy fuel. Low = hypoglycemia (diabetes on insulin, sepsis) — risk of seizure; high = hyperglycemia (diabetes) — risk of ketoacidosis. Normal fasting ~70–100. (6) Creatinine (mg/dL): kidney function marker. High = kidney disease (nephritis, obstruction, diabetes). Normal ~0.7–1.3 men, ~0.6–1.1 women. (7) BUN (blood urea nitrogen, mg/dL): kidney function marker. High = kidney disease, dehydration. Normal ~7–20. Creatinine and BUN are usually evaluated together; high both = kidney disease, high BUN/low creatinine = dehydration. A BMP gives a quick snapshot of electrolyte balance and organ function, making it the most commonly ordered chemistry test.

Serum is blood that has clotted and had the clot removed (red-top tube, no anticoagulant). Plasma is blood to which an anticoagulant (EDTA, heparin, sodium citrate) has been added, preventing clotting (purple, green, light-blue tubes). Whole blood is unprocessed blood used for some point-of-care testing (glucose meters, blood gas analyzers). The difference matters because: (1) Some tests require serum (chemistry, most immunology); others require plasma (coagulation studies use sodium citrate plasma; hematology uses EDTA plasma). (2) Chemistry analyzers are optimized for one sample type; using the wrong type (e.g., EDTA plasma for chemistry) causes falsely low results because EDTA chelates divalent cations like calcium. (3) Clotting takes time (30 minutes), so serum tubes must sit in the phlebotomist's hand or a tube holder; delays lead to hemolysis or hemoconcentration. (4) Additives affect results: EDTA lowers potassium and ionized calcium; heparin may interfere with some tests. Specimen type is specified on the requisition, and collecting in the wrong tube wastes the patient's stick and delays results. Proper specimen handling (right tube, right timing, gentle mixing, proper storage temperature) is the foundation of valid results.

These three conditions degrade specimen quality and interfere with chemistry analyzer readings (they absorb or scatter light in optical detection). (1) Hemolysis: rupture of RBCs, releasing hemoglobin and intracellular contents. Cause: small-gauge needle (vacuum is too strong), vigorous tube shaking, prolonged tourniquet time (>1 minute), drawing from a hematoma, or delayed processing. Effects: falsely elevates potassium (released from RBCs), LDH, AST, ALT, bilirubin, and phosphorus; gives serum pink/red color. (2) Lipemia: excess lipids (triglycerides, cholesterol) in serum, making it cloudy white. Cause: non-fasting state, high triglycerides (>400 mg/dL), liver disease, nephrotic syndrome (loses lipoproteins in urine but reabsorbs them). Effects: falsely elevates bilirubin, phosphorus, and decreases some analytes (sodium measured by ion-selective electrode may be falsely low because sodium concentration in aqueous phase is lower). (3) Icterus: excess bilirubin (yellow pigmentation), seen in jaundice. Cause: hemolysis, liver disease, cholestasis (blocked bile ducts). Effects: falsely elevates bilirubin (obviously), and may interfere with other optical measurements. Modern analyzers have automated detection of hemolysis, lipemia, and icterus and will flag results as 'unsuitable' or comment the result. If flagged, the tech must request a redraw (for hemolysis) or, in some cases, use dilution or mathematical correction. Understanding these interferences prevents false results and unnecessary clinical decisions.

The anion gap (AG) is a calculated value that represents unmeasured anions (negatively charged ions) in blood. Formula: AG = Na − (Cl + CO2), where Na = sodium, Cl = chloride, CO2 = bicarbonate. Normal AG = 8–16 mEq/L. The concept: sodium is balanced by anions; if measured anions (Cl and CO2) don't account for all sodium, unmeasured anions (phosphate, sulfate, protein, lactate, ketones) must be present. A high AG indicates unmeasured anions and narrows the differential of metabolic acidosis: High AG metabolic acidosis = accumulation of organic acids (lactate, ketones, uremia) or ingestion of acid (salicylates, methanol, ethylene glycol). Mnemonic MUDPILES: Methanol, Uremia, Diabetic ketoacidosis (DKA), Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates. Normal AG metabolic acidosis = loss of bicarbonate or retention of hydrogen ions; causes include diarrhea (bicarbonate loss), renal tubular acidosis (kidney can't acidify urine). In clinical practice: a BMP shows low CO2 (acidosis). Calculate AG. If high AG, suspect DKA, lactic acidosis, or toxin; if normal AG, suspect diarrhea or renal disease. Correct interpretation guides clinical management (insulin for DKA, fluid resuscitation for lactic acidosis, specific antidotes for toxins).

These four markers assess different aspects of liver health. (1) AST (aspartate aminotransferase, U/L): enzyme released from liver and muscle when cells are damaged. Elevated = hepatitis (viral, alcoholic, autoimmune), cirrhosis, sepsis, myocardial infarction, muscle injury. Non-specific. (2) ALT (alanine aminotransferase, U/L): liver-specific enzyme (also in muscle, kidney). Elevated = hepatitis (more specific than AST for liver), cirrhosis, fatty liver disease. More specific than AST. (3) AST:ALT ratio: AST > ALT suggests alcoholic liver disease or cirrhosis; ALT > AST suggests viral hepatitis or fatty liver. (4) Bilirubin (mg/dL): breakdown product of hemoglobin. Processed by liver. Elevated = hemolysis (unconjugated), liver disease (can't process bilirubin = conjugated), or bile duct obstruction (conjugated bilirubin backs up into blood). Total bilirubin = unconjugated + conjugated. Conjugated (direct) > 50% of total = cholestasis (liver can't excrete). (5) Albumin (g/dL): protein made by liver. Low = liver disease (can't synthesize), kidney disease (loss in urine), malnutrition, inflammation. Albumin is a marker of liver synthetic function and nutritional status. Interpretation: mild elevation of AST/ALT with normal bilirubin and albumin = hepatitis (reversible). High bilirubin and low albumin = cirrhosis or fulminant failure (irreversible). High alkaline phosphatase and high bilirubin = cholestasis. Understanding these markers and their patterns guides diagnosis of liver disease type and severity.

Glucose and hemoglobin A1C both assess glucose metabolism but over different timescales. Glucose (mg/dL, fasting or random): blood sugar at the moment of testing. Normal fasting <100, normal random <140, diabetes fasting ≥126. Used for diagnosis of diabetes and acute management of hyperglycemia/hypoglycemia. A1C (% or mmol/mol): also called glycated hemoglobin, measures the percentage of hemoglobin molecules that have glucose attached (glycation). This occurs over ~120 days (red cell lifespan), so A1C reflects average glucose over the past 2–3 months. Used for long-term glucose control and diabetes monitoring. Normal A1C <5.7%, prediabetes 5.7–6.4%, diabetes ≥6.5%. A1C advantages: not affected by acute glucose spikes (fasting/post-meal variability), doesn't require fasting, better reflects long-term control and risk of complications. Glucose advantages: immediate assessment of acute state, used for diagnosis and acute treatment. Interpretation: a patient can have normal glucose but elevated A1C (indicating poor control over past 2–3 months), or vice versa (glucose high today but A1C normal if control was good overall). A1C >7% indicates inadequate diabetes control and need for medication adjustment. Conditions that affect A1C: hemolysis (low A1C because RBCs live shorter), anemia, pregnancy. A1C is more specific for true glucose control than any single glucose measurement.