Lab Interpretation

Anemia Workup Algorithm

1. Confirm anemia (Hb below reference for age/sex)
2. Check MCV to classify morphology
3. Microcytic: ferritin (iron deficiency vs thalassemia vs chronic disease vs sideroblastic)
4. Normocytic: reticulocyte count (hemolysis/bleeding vs underproduction)
5. Macrocytic: B12/folate, reticulocyte count, peripheral smear (megaloblastic vs non-megaloblastic)

White Cell Parameters

• Leukocytosis (>11,000/uL): Infection, inflammation, stress, steroids, malignancy, smoking
• Leukopenia (<4,000/uL): Viral infection, drug effect, autoimmune, bone marrow failure
• Differential matters: Neutrophilia (bacterial), lymphocytosis (viral, CLL), eosinophilia (allergy, parasites, drugs), monocytosis (chronic inflammation, recovery)
• Left shift: Increased bands (>6%) suggests acute bacterial infection or severe stress
• Neutropenia grading: Mild 1000-1500, moderate 500-1000, severe <500 (infection risk escalates)

Platelet Parameters

• Thrombocytopenia (<150,000/uL): Decreased production (marrow), increased destruction (ITP, TTP/HUS, DIC, HIT), sequestration (splenomegaly), pseudothrombocytopenia (EDTA clumping — always check smear)
• Thrombocytosis (>400,000/uL): Reactive (iron deficiency, inflammation, infection) vs clonal (myeloproliferative neoplasms)

Basic/Comprehensive Metabolic Panel (BMP/CMP)

Electrolytes

• Sodium: Hyponatremia — assess volume status first (hypovolemic, euvolemic, hypervolemic). Check serum osmolality (true vs pseudo). SIADH criteria: euvolemic, low serum osm, inappropriately concentrated urine (>100 mOsm), urine Na >40.
• Potassium: Hypokalemia — check magnesium (must replete Mg to correct K). Hyperkalemia — rule out hemolysis artifact, check ECG immediately if >6.0 mEq/L.
• Calcium: Always correct for albumin: corrected Ca = measured Ca + 0.8 x (4.0 - albumin). If abnormal, check ionized calcium, PTH, 25-OH vitamin D, phosphorus.
• Magnesium: Often under-ordered. Check in refractory hypokalemia, hypocalcemia, arrhythmias, seizures.

Renal Function

• Creatinine: Influenced by muscle mass, diet, medications (trimethoprim, cimetidine block tubular secretion). Estimate GFR with CKD-EPI equation, not Cockcroft-Gault for staging.
• BUN: BUN:Cr ratio >20:1 suggests prerenal azotemia or upper GI bleeding.
• BUN and Cr both rising: Distinguish prerenal (reversible with fluids) vs intrinsic renal (ATN — fractional excretion of sodium >2%) vs postrenal (obstruction — get ultrasound).

Hepatic Panel (CMP)

• AST/ALT: Hepatocellular pattern. ALT more liver-specific. AST:ALT >2:1 suggests alcoholic liver disease. Massive elevation (>1000) — viral hepatitis, ischemic hepatitis, drug toxicity (acetaminophen).
• Alkaline phosphatase: Cholestatic pattern. Confirm hepatic origin with GGT (bone ALP is GGT-negative).
• Bilirubin: Direct (conjugated) vs indirect (unconjugated). Isolated indirect hyperbilirubinemia — hemolysis or Gilbert syndrome. Direct hyperbilirubinemia — hepatobiliary obstruction or hepatocellular dysfunction.
• Albumin: Marker of synthetic function and nutritional status. Half-life ~20 days — not useful for acute changes.

Coagulation Studies

• PT/INR: Extrinsic pathway (factors VII, X, V, II, fibrinogen). Monitors warfarin. Elevated in liver disease, vitamin K deficiency, DIC.
• aPTT: Intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, fibrinogen). Monitors unfractionated heparin. Elevated in hemophilia A (VIII) and B (IX), lupus anticoagulant (paradoxically causes thrombosis).
• Mixing study: If prolonged PT/aPTT corrects with 1:1 mixing — factor deficiency. If it does not correct — inhibitor present.
• Fibrinogen: Acute phase reactant (rises in inflammation). Low in DIC, severe liver disease, massive transfusion.
• D-dimer: Fibrin degradation product. High sensitivity, low specificity. Useful to rule OUT PE/DVT (high NPV). Age-adjusted cutoff: age x 10 ug/L for patients >50 years.

Cardiac Markers

• Troponin (high-sensitivity): Gold standard for myocardial injury. Rise within 1-3 hours. Use 0/1h or 0/3h rule-in/rule-out algorithms per ESC guidelines. Remember: troponin elevation ≠ MI. Type 2 MI (supply-demand mismatch), myocarditis, PE, renal failure, sepsis all elevate troponin.
• BNP/NT-proBNP: Heart failure biomarkers. BNP <100 pg/mL or NT-proBNP <300 pg/mL effectively rules out acute HF. Elevated in renal failure, AF, PE, sepsis. NT-proBNP is not cleared by neprilysin — preferred if patient on sacubitril/valsartan.

Thyroid Function

• TSH-first strategy: TSH is the most sensitive screening test for thyroid dysfunction.
• TSH high, fT4 low: Primary hypothyroidism
• TSH low, fT4 high: Primary hyperthyroidism (next: T3, TSH receptor antibodies, thyroid uptake scan)
• TSH low, fT4 normal: Subclinical hyperthyroidism or sick euthyroid (check clinical context)
• TSH normal-low, fT4 low: Central hypothyroidism (pituitary) — do not rely on TSH alone
• Sick euthyroid (non-thyroidal illness): Low T3, variable TSH — avoid testing thyroid function in acutely ill patients unless strong clinical suspicion

Inflammatory Markers

• CRP: Non-specific. Rises within 6-8 hours, peaks 48 hours. Useful for monitoring treatment response. hs-CRP for cardiovascular risk stratification.
• Procalcitonin (PCT): More specific for bacterial infection than CRP. PCT <0.25 ng/mL — bacterial infection unlikely (consider stopping/withholding antibiotics in lower respiratory infections). PCT >0.5 ng/mL — bacterial infection likely. Useful for antibiotic stewardship.
• ESR: Very non-specific. Rises slowly. Elevated in infection, inflammation, malignancy, pregnancy, anemia. Temporal arteritis (ESR >50 mm/hr with clinical features) and polymyalgia rheumatica are classic indications.

Urinalysis

• Dipstick: pH, specific gravity, protein, glucose, ketones, blood, leukocyte esterase, nitrites, bilirubin, urobilinogen
• Leukocyte esterase + nitrites: Together have high specificity for UTI. Nitrites only positive with nitrate-reducing organisms (Enterobacteriaceae — not Enterococcus, Pseudomonas, or Staphylococcus).
• Proteinuria: Dipstick detects albumin only. Quantify with spot urine albumin:creatinine ratio (ACR) or protein:creatinine ratio (PCR). ACR >30 mg/g = albuminuria.
• Microscopy: RBC casts (glomerulonephritis), WBC casts (pyelonephritis/interstitial nephritis), muddy brown granular casts (ATN), hyaline casts (nonspecific), waxy/broad casts (CKD)

Interpretation Framework

When presented with lab results, follow this sequence:

1. Flag critical values requiring immediate action (K >6.5, Na <120, glucose <40, troponin elevation, lactate >4)
2. Identify patterns across related tests (hepatocellular vs cholestatic, DIC triad, tumor lysis)
3. Correlate with clinical picture (pre-test probability changes interpretation)
4. Recommend reflex tests (e.g., iron studies for microcytic anemia, peripheral smear for unexplained cytopenias)
5. Suggest monitoring interval based on trajectory and clinical stability

Common Pitfalls

• Treating the number, not the patient
• Ignoring hemolysis flags (falsely elevates K, LDH, AST)
• Forgetting albumin correction for calcium
• Ordering thyroid function tests in acutely ill patients
• Using D-dimer in high pre-test probability patients (order imaging directly)
• Interpreting a single troponin without serial measurement or clinical context