Lab Values & Diagnostics Memory Tricks | A&P II

🧪 Lab Values & Diagnostics

Memory Tricks

Proven Mnemonics & Acronyms — fast to learn, hard to forget.

Complete Blood Count (CBC)

WBC 4.5–11 · RBC 4.5–5.5 M · Hgb 12–17 · Hct 37–52% · Platelets 150–400K

Five key CBC values — each tells a different story about blood cell production

CBC normal ranges — and what high or low values indicate

WBC (4,500–11,000/µL): ↑ = infection, inflammation, leukemia. ↓ = bone marrow suppression, viral illness, chemotherapy. Differential: neutrophils (55–70%), lymphocytes (20–40%), monocytes (2–8%), eosinophils (1–4%), basophils (0.5–1%). RBC (4.5–5.5 million/µL M, 4.0–5.0 F): ↓ = anemia. ↑ = polycythemia vera, dehydration. Hemoglobin (12–17 g/dL): ↓ = anemia (pale, fatigue, dyspnea). Hematocrit (37–52%): ≈ 3× hemoglobin. Platelets (150,000–400,000/µL): ↓ = thrombocytopenia → bleeding risk. ↑ = thrombocytosis → clotting risk. MCV (80–100 fL): ↓ = microcytic (iron deficiency), ↑ = macrocytic (B12/folate deficiency).

WBC ↑

Leukocytosis — bacterial infection, inflammation, stress, leukemia. Left shift = immature neutrophils.

Hgb ↓

Anemia — fatigue, pallor, dyspnea on exertion. Transfuse if <7 g/dL (symptomatic).

Platelets ↓

Below 50K → spontaneous bleeding risk. Below 10K → life-threatening. Cause: ITP, HIT, DIC, chemo.

MCV

Mean corpuscular volume. Small (microcytic, <80) = iron, thalassemia. Large (macrocytic, >100) = B12/folate.

📖 Full Lesson

Basic Metabolic Panel (BMP)

SUCKS BP — Sodium · Urea · Creatinine · K+ · Sugar · Bicarb · pH

Seven core electrolyte and metabolic values — kidney, glucose, and acid-base at a glance

BMP normal ranges — electrolytes, kidney function, and glucose

Sodium (135–145 mEq/L): ↓ hyponatremia (SIADH, heart failure, over-hydration) → confusion, seizures. ↑ hypernatremia (dehydration, DI) → thirst, dry mucous membranes. Potassium (3.5–5.0 mEq/L): ↓ hypokalemia → muscle weakness, arrhythmias (U wave on ECG). ↑ hyperkalemia → peaked T waves → cardiac arrest. Creatinine (0.6–1.2 mg/dL): kidney waste product — best simple kidney marker. ↑ = kidney disease or decreased GFR. BUN (7–20 mg/dL): urea nitrogen. BUN:Cr ratio >20 = prerenal. Glucose (70–100 fasting): ↑ = diabetes. ↓ = hypoglycemia → tremor, diaphoresis, confusion. Bicarbonate (22–29 mEq/L): metabolic acid-base marker.

Na+ 135–145

Hyponatremia → brain swelling → confusion, seizures. Hypernatremia → brain shrinkage → irritability, coma.

K+ 3.5–5.0

Most critical electrolyte for cardiac rhythm. Hypo = U waves. Hyper = peaked T → VFib.

Creatinine 0.6–1.2

Muscle waste product filtered by kidney. Doubles when GFR halves. Better than BUN alone.

BUN:Cr ratio

>20 = prerenal (dehydration). <10 = intrinsic renal. 10–20 = normal or mixed.

📖 Full Lesson

Arterial Blood Gas (ABG)

pH 7.35–7.45 · PaCO₂ 35–45 · PaO₂ 80–100 · HCO₃⁻ 22–26 · SpO₂ >95%

Five ABG values — ROME helps interpret: Respiratory Opposite, Metabolic Equal

ABG interpretation — four-step approach using ROME

Step 1 — pH: <7.35 = acidosis. >7.45 = alkalosis. Step 2 — Respiratory (PaCO₂): ↑ CO₂ = more acid → respiratory acidosis. ↓ CO₂ = more alkaline → respiratory alkalosis. CO₂ moves OPPOSITE to pH (ROME-R). Step 3 — Metabolic (HCO₃⁻): ↑ HCO₃⁻ = more alkaline → metabolic alkalosis. ↓ HCO₃⁻ = more acid → metabolic acidosis. HCO₃⁻ moves SAME direction as pH (ROME-M). Step 4 — Compensation: Lungs compensate metabolic disorders (minutes). Kidneys compensate respiratory disorders (days). PaO₂ 80–100 mmHg: <60 = hypoxemia → supplemental O₂ needed. SpO₂ = peripheral O₂ saturation (pulse oximeter) — normal >95%.

ROME

Respiratory Opposite (CO₂ up = pH down), Metabolic Equal (HCO₃⁻ up = pH up).

Resp. acidosis

↓ pH, ↑ CO₂. COPD, opioid OD, hypoventilation. Kidneys compensate: ↑ HCO₃⁻.

Met. acidosis

↓ pH, ↓ HCO₃⁻. DKA, renal failure, lactic acidosis. Lungs compensate: ↓ CO₂ (Kussmaul breathing).

PaO₂ <60

Hypoxemia — stimulates peripheral chemoreceptors. SpO₂ <90% = critical. CO poisoning = normal SpO₂!

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Cardiac Markers

Troponin I/T rises in 3–6 hrs · peaks 12–24 hrs · stays 7–14 days — most specific for MI

Troponin (most specific) · CK-MB (early) · BNP (heart failure) · LDH (late)

Cardiac biomarkers — timing of rise, peak, and fall after myocardial infarction

Troponin I and T: most sensitive and specific cardiac marker. Rises 3–6 hours post-MI, peaks 12–24 hours, remains elevated 7–14 days. High-sensitivity troponin (hs-cTn) detectable within 1 hour. Any elevation = myocardial injury. CK-MB (creatine kinase MB isoform): rises 4–6 hours, peaks 12–24 hours, returns to normal 2–3 days. Useful for detecting reinfarction (troponin stays elevated). BNP / NT-proBNP: released by ventricles when stretched. ↑ = heart failure. Normal BNP <100 pg/mL makes heart failure unlikely. Used to diagnose and monitor CHF. Myoglobin: earliest marker (1–2 hours) but not specific to heart. LDH: late marker, peaks 3–5 days, stays elevated 10 days — historical.

Troponin

Gold standard for MI. Rises 3–6 hrs, peaks 24 hrs, elevated 7–14 days. Any rise = myocardial injury.

CK-MB

Back to normal in 2–3 days — useful to detect reinfarction when troponin still elevated.

BNP

B-type natriuretic peptide. ↑ in heart failure. <100 = HF unlikely. >500 = HF very likely.

ECG changes

ST elevation → STEMI (emergent PCI). ST depression → NSTEMI/UA. T wave inversion → ischemia.

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Liver Function Tests (LFTs)

ALT and AST = hepatocyte damage · ALP + GGT = cholestasis · Bilirubin + Albumin = liver function

Hepatocellular injury vs cholestasis vs synthetic function — three different problems

LFT interpretation — what each value reveals about liver health

ALT (alanine aminotransferase, 7–56 U/L): most specific for liver — rises in hepatocyte damage (hepatitis, cirrhosis, fatty liver, drug toxicity). AST (aspartate aminotransferase, 10–40 U/L): less specific (also heart, muscle). AST:ALT ratio >2:1 = alcoholic hepatitis. ALP (alkaline phosphatase, 44–147 U/L): ↑ = cholestasis (bile duct obstruction), bone disease, pregnancy. GGT confirms biliary source. Bilirubin (0.1–1.2 mg/dL total): unconjugated (indirect) ↑ = hemolysis or liver uptake failure. Conjugated (direct) ↑ = hepatic or post-hepatic obstruction. Jaundice visible >2.5 mg/dL. Albumin (3.5–5.0 g/dL): synthetic function — ↓ = liver failure, malnutrition. PT/INR: clotting factor synthesis — ↑ INR = liver failure or warfarin.

ALT > AST

Viral hepatitis, NAFLD. ALT is more liver-specific.

AST:ALT >2

Alcoholic hepatitis — mitochondrial AST released. GGT also markedly elevated in alcohol use.

↑ ALP + GGT

Cholestasis — gallstones, primary biliary cholangitis, PSC, drug-induced. ALP alone = also bone.

Albumin + PT

Best markers of liver FUNCTION (synthetic ability). Low albumin + high PT = liver failure.

📖 Full Lesson

Renal Function Tests

Creatinine 0.6–1.2 · BUN 7–20 · eGFR >60 · BUN:Cr ratio tells the story

Creatinine and BUN reflect GFR · eGFR estimates kidney function · ratio localizes problem

Renal lab values — how to assess kidney function and locate the problem

Creatinine (0.6–1.2 mg/dL): muscle waste product, freely filtered — best simple GFR estimate. Doubles when GFR halves. Rises slowly — takes 24–48 hours to reflect acute injury. BUN (7–20 mg/dL): blood urea nitrogen — affected by protein intake, hydration, GI bleeding. eGFR (estimated GFR): calculated from creatinine + age + sex + race. Normal >60 mL/min. CKD defined as eGFR <60 for >3 months. BUN:Creatinine ratio: >20 = prerenal (dehydration, GI bleeding, heart failure — kidney OK but underperfused). <10 = intrinsic renal (ATN, glomerulonephritis). 10–20 = normal. Urine sodium: <20 mEq/L = prerenal (kidney conserving Na+). >40 mEq/L = intrinsic renal (tubules can't reabsorb). FENa = fractional excretion of sodium — <1% prerenal, >2% intrinsic.

Creatinine doubles

When GFR halves — e.g., Cr 1.2 → 2.4 means GFR fell from 60 → 30 mL/min.

eGFR staging

CKD G1 >90, G2 60–89, G3 30–59, G4 15–29, G5 <15 (kidney failure, dialysis).

BUN:Cr >20

Prerenal — dehydration, heart failure, GI bleed (blood = protein = ↑ BUN). Kidneys intact.

FENa <1%

Prerenal — kidney avidly holding Na+. FENa >2% = ATN (tubules damaged, can't reabsorb).

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Thyroid Function Tests

TSH is the gatekeeper — high TSH = hypothyroid · low TSH = hyperthyroid · T4 confirms

TSH 0.4–4.0 mU/L · Free T4 0.8–1.8 ng/dL · T3 80–200 ng/dL

Thyroid labs — TSH is the best screening test and how to interpret the pattern

TSH (thyroid-stimulating hormone, 0.4–4.0 mU/L): most sensitive thyroid test — reflects pituitary response to T3/T4. High TSH = pituitary working hard to stimulate failing thyroid = hypothyroidism. Low TSH = pituitary suppressed by excess thyroid hormone = hyperthyroidism. Free T4 (0.8–1.8 ng/dL): unbound active form. Confirms and grades severity. Primary hypothyroidism: ↑ TSH + ↓ T4. Treat with levothyroxine. Primary hyperthyroidism: ↓ TSH + ↑ T4. Graves' disease most common. Secondary hypothyroidism (pituitary failure): ↓ TSH + ↓ T4. Subclinical hypothyroidism: ↑ TSH + normal T4 — treat if TSH >10 or symptomatic.

↑ TSH + ↓ T4

Primary hypothyroidism — Hashimoto's most common. Treat: levothyroxine (T4 replacement).

↓ TSH + ↑ T4

Primary hyperthyroidism — Graves' disease, toxic nodule. Treat: methimazole, RAI, surgery.

↓ TSH + ↓ T4

Secondary (central) hypothyroidism — pituitary or hypothalamic failure. Check other pituitary hormones.

Symptoms guide

Hypo: cold, slow, constipated, weight↑, bradycardia. Hyper: hot, fast, diarrhea, weight↓, tachycardia.

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Coagulation Studies

PT/INR = extrinsic (warfarin) · PTT = intrinsic (heparin) · Both ↑ = DIC or liver failure

PT 11–13 sec · INR 0.8–1.1 · PTT 25–35 sec · Fibrinogen · D-dimer

Coagulation labs — which test checks which pathway and what drugs affect each

PT (prothrombin time, 11–13 sec) / INR (0.8–1.1): tests extrinsic pathway (factor VII) and common pathway. Prolonged by: warfarin (blocks vitamin K-dependent factors VII, IX, X, II), liver failure (can't make factors), vitamin K deficiency. INR 2–3 = therapeutic warfarin range for most indications. PTT (partial thromboplastin time, 25–35 sec): tests intrinsic pathway and common pathway. Prolonged by: heparin, hemophilia A (factor VIII) or B (factor IX), lupus anticoagulant. Therapeutic heparin PTT = 60–100 sec. Both prolonged: liver failure, DIC, massive transfusion. D-dimer (normal <0.5 µg/mL): fibrin degradation product → ↑ in DVT/PE, DIC, any clot. High sensitivity but low specificity. Normal D-dimer rules out PE (high sensitivity).

PT/INR ↑

Warfarin effect, liver failure, vitamin K deficiency (newborns, malabsorption, antibiotics).

PTT ↑

Heparin, hemophilia A/B, lupus anticoagulant (paradoxically causes clotting in vivo).

DIC

Both PT and PTT ↑ + ↓ platelets + ↑ D-dimer + ↓ fibrinogen = disseminated intravascular coagulation.

D-dimer

High sensitivity for VTE — negative result rules out PE/DVT. Low specificity — positive in many conditions.

📖 Full Lesson

Glucose and Diabetes Labs

Fasting glucose 70–100 · HbA1c <5.7% normal · 5.7–6.4% prediabetes · ≥6.5% diabetes

HbA1c reflects average blood glucose over 3 months — one number tells the whole story

Diabetes diagnostic labs — fasting glucose, HbA1c, and what they mean

Fasting plasma glucose: Normal <100 mg/dL. Prediabetes 100–125 mg/dL. Diabetes ≥126 mg/dL (confirmed on 2 occasions). Random glucose ≥200 mg/dL with symptoms = diabetes. 2-hour OGTT (oral glucose tolerance test): normal <140 mg/dL. Prediabetes 140–199 mg/dL. Diabetes ≥200 mg/dL. HbA1c (glycated hemoglobin): reflects average glucose for ~3 months (RBC lifespan). Normal <5.7%. Prediabetes 5.7–6.4%. Diabetes ≥6.5%. Target for diabetics: <7% (ADA). Each 1% HbA1c ≈ average glucose of 28 mg/dL. Insulin and C-peptide: ↑ both = Type 2 or insulinoma. ↓ both = Type 1 (no insulin production). ↑ insulin + ↓ C-peptide = exogenous insulin injection (factitious hypoglycemia).

HbA1c 6.5%

Diagnostic for diabetes. Equivalent to average glucose ~126 mg/dL. No fasting required.

HbA1c <7%

ADA target for most diabetics. Each 1% reduction → 35% ↓ microvascular complications.

C-peptide

Proinsulin cleavage product — marker of endogenous insulin production. Low in T1DM. High in T2DM/insulinoma.

DKA labs

High glucose + high ketones + metabolic acidosis (↓ pH, ↓ HCO₃⁻, ↑ anion gap). T1DM emergency.

📖 Full Lesson

🎓 Common Exam Questions

Q: What are the normal values for an arterial blood gas (ABG) and how do you interpret one?

A: Normal: pH 7.35-7.45, pCO2 35-45 mmHg, HCO3- 22-26 mEq/L, pO2 80-100 mmHg, SaO2 >95%. Step 1: Is pH acidic (<7.35) or alkaline (>7.45)? Step 2: Primary disorder — pCO2 high = respiratory acidosis; pCO2 low = respiratory alkalosis; HCO3- low = metabolic acidosis; HCO3- high = metabolic alkalosis. Rule: the primary disorder matches the pH change. Step 3: Is compensation appropriate? Winter formula (metabolic acidosis): expected pCO2 = 1.5 × HCO3- + 8 ± 2. If actual pCO2 higher → additional respiratory acidosis. Step 4: Anion gap = Na - (Cl + HCO3-), normal 8-12. Elevated AG metabolic acidosis = MUDPILES: Methanol, Uremia, DKA, Propylene glycol/Paracetamol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.

Q: What are the key values in a Complete Blood Count (CBC) and what do abnormalities indicate?

A: WBC: 4,500-11,000/μL. Neutrophilia: bacterial infection, steroids. Lymphocytosis: viral infection, CLL. Eosinophilia: parasites, allergy, asthma (NAACP: Neoplasm, Asthma, Allergy, Collagen vascular, Parasites). RBC/Hgb/Hct: men 13.5-17.5 g/dL Hgb; women 12-15.5. Anemia workup: MCV low → iron deficiency (most common), thalassemia; MCV normal → acute blood loss, chronic disease; MCV high → B12/folate deficiency, alcohol, hypothyroidism. Platelets: 150,000-400,000/μL. Thrombocytopenia (<150k): ITP (autoimmune), TTP, HIT, DIC, hypersplenism. Thrombocytosis: reactive (infection, iron deficiency) vs essential thrombocythemia.

Q: What do elevated troponin, BNP, and D-dimer indicate?

A: Troponin I and T: cardiac myocyte damage markers. Rise 3-6 hours after MI, peak 12-24h, normalize 7-10 days. Most sensitive and specific for myocardial infarction. Also elevated in: demand ischemia, myocarditis, PE, renal failure (decreased clearance). High-sensitivity troponin allows earlier detection. BNP/NT-proBNP: released by stretched ventricles. BNP >100 pg/mL suggests heart failure. Used to distinguish cardiac from pulmonary dyspnea (elevated in HF, normal in COPD exacerbation). NT-proBNP has longer half-life. D-dimer: fibrin degradation product — elevated when clot being broken down. Sensitive but not specific for PE/DVT. Negative D-dimer in low-probability patient effectively rules out PE (Wells criteria).

Q: What are the liver function tests and what pattern indicates hepatocellular vs cholestatic disease?

A: Hepatocellular pattern: markedly elevated AST and ALT (transaminases) with mild ALP elevation. AST:ALT ratio >2:1 suggests alcoholic hepatitis (AST >500 suggests other cause). Causes: viral hepatitis, NAFLD, autoimmune, ischemic, drug toxicity. Cholestatic pattern: markedly elevated ALP and GGT with mild AST/ALT elevation. Causes: bile duct obstruction (gallstones, cancer), primary biliary cholangitis, PSC, drug-induced. Synthetic function: albumin (half-life 20 days — reflects chronic function), PT/INR (reflects acute function — clotting factors made by liver have shorter half-lives). Bilirubin: unconjugated elevated in hemolysis or Gilbert syndrome; conjugated elevated in liver disease or biliary obstruction (conjugated bilirubin is water-soluble → urine turns dark, stools pale).

Q: What coagulation tests measure what and how do you interpret bleeding disorders?

A: PT/INR: extrinsic pathway (Factor VII) and common pathway (X, V, II, I). Prolonged by: warfarin (inhibits vitamin K-dependent factors II, VII, IX, X), liver disease, vitamin K deficiency, DIC. Normal: PT 11-13 sec, INR 0.8-1.2. aPTT: intrinsic pathway (XII, XI, IX, VIII) and common pathway. Prolonged by: heparin, hemophilia A (Factor VIII deficiency), hemophilia B (Factor IX deficiency), lupus anticoagulant. Normal: 25-35 sec. Bleeding time/PFA: platelet function. Elevated in: thrombocytopenia, VWD, aspirin, uremia. DIC: prolonged PT + aPTT + low platelets + low fibrinogen + elevated D-dimer + schistocytes on smear. VWD: most common inherited bleeding disorder. Factor VIII low (VWF carries it), prolonged aPTT + bleeding time.

Memory tricks for normal ranges and what they mean

Memory Tricks