Lab values are tested frequently on the NCLEX-RN. You will need to recognize normal ranges, understand what abnormal values mean clinically, and know the appropriate nursing response. This reference covers the most commonly tested lab values organized by category.
Electrolytes
Electrolyte imbalances are a high-yield NCLEX topic. Know the normal ranges and the signs and symptoms of both high and low levels.
| Lab Test | Normal Range | Low (Key Signs) | High (Key Signs) |
|---|---|---|---|
| Sodium (Na+) | 136-145 mEq/L | Confusion, seizures, nausea, headache | Thirst, restlessness, edema, elevated BP |
| Potassium (K+) | 3.5-5.0 mEq/L | Muscle weakness, arrhythmias, flat T waves, leg cramps | Peaked T waves, arrhythmias, muscle twitching, cardiac arrest |
| Calcium (Ca2+) | 9.0-10.5 mg/dL | Trousseau sign, Chvostek sign, numbness, tetany, seizures | Muscle weakness, kidney stones, confusion, constipation |
| Magnesium (Mg2+) | 1.3-2.1 mEq/L | Tremors, seizures, hyperactive reflexes, arrhythmias | Hypotension, decreased reflexes, respiratory depression |
| Phosphorus (PO4) | 3.0-4.5 mg/dL | Muscle weakness, confusion, bone pain | Tetany, muscle cramps (often inverse of calcium) |
| Chloride (Cl-) | 98-106 mEq/L | Muscle spasms, metabolic alkalosis | Weakness, metabolic acidosis, deep rapid breathing |
Key relationship: Calcium and phosphorus have an inverse relationship. When one goes up, the other goes down. Potassium and digoxin interact dangerously: hypokalemia increases digoxin toxicity risk.
Complete Blood Count (CBC)
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| WBC (White Blood Cells) | 5,000-10,000/mcL | Elevated: infection, inflammation, leukemia. Low: immunosuppression, risk of infection. |
| RBC (Red Blood Cells) | M: 4.7-6.1 million/mcL F: 4.2-5.4 million/mcL |
Low: anemia, bleeding. Elevated: polycythemia, dehydration. |
| Hemoglobin (Hgb) | M: 14-18 g/dL F: 12-16 g/dL |
Oxygen-carrying capacity. Low: anemia, bleeding. Critical if below 7 g/dL (transfusion may be needed). |
| Hematocrit (Hct) | M: 42-52% F: 37-47% |
Percentage of blood volume that is RBCs. Elevated in dehydration, low in anemia/bleeding. |
| Platelets | 150,000-400,000/mcL | Low (thrombocytopenia): bleeding risk. Below 50,000: spontaneous bleeding risk. Below 20,000: life-threatening. |
| Neutrophils | 55-70% of WBC | First responders to bacterial infection. Absolute neutrophil count below 500: severe infection risk (neutropenic precautions). |
Coagulation Studies
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| PT (Prothrombin Time) | 11-12.5 seconds | Monitors warfarin therapy. Prolonged = bleeding risk. |
| INR | 0.8-1.1 (normal) 2.0-3.0 (on warfarin) 2.5-3.5 (mechanical valve) |
Standardized measure for warfarin. Above therapeutic range = bleeding risk. Below = clotting risk. |
| aPTT | 30-40 seconds 1.5-2.5x normal (on heparin) |
Monitors heparin therapy. Prolonged = bleeding risk. |
| D-dimer | <500 ng/mL | Elevated in DVT, PE, DIC. Useful for ruling out thromboembolism. |
| Fibrinogen | 200-400 mg/dL | Low in DIC, liver disease. Essential for clot formation. |
Memory tip: PT/INR monitors warfarin (both have letters "r" in common — Pro-th-R-ombin, wa-R-fa-R-in). aPTT monitors heparin (think "Partial = Par-heparin").
Metabolic Panel / Renal Function
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| BUN (Blood Urea Nitrogen) | 10-20 mg/dL | Kidney function indicator. Elevated in dehydration, kidney disease, GI bleeding, high-protein diet. |
| Creatinine | 0.7-1.3 mg/dL | More specific kidney function indicator than BUN. Elevated = decreased kidney function. |
| GFR | >90 mL/min (normal) | Best overall indicator of kidney function. Below 60 = chronic kidney disease. Below 15 = kidney failure. |
| Uric Acid | M: 3.4-7.0 mg/dL F: 2.4-6.0 mg/dL |
Elevated in gout, kidney disease, tumor lysis syndrome. |
| Albumin | 3.5-5.0 g/dL | Nutritional status indicator. Low in malnutrition, liver disease, nephrotic syndrome. Affects drug binding and edema. |
| Total Protein | 6.0-8.3 g/dL | Low in malnutrition, liver disease. Elevated in dehydration, multiple myeloma. |
Liver Function Tests
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| ALT (Alanine Aminotransferase) | 7-56 U/L | Most specific liver enzyme. Elevated in hepatitis, liver damage, hepatotoxic drugs. |
| AST (Aspartate Aminotransferase) | 10-40 U/L | Elevated in liver damage, MI, muscle injury. Less specific than ALT for liver. |
| ALP (Alkaline Phosphatase) | 44-147 U/L | Elevated in bile duct obstruction, bone disease, liver disease. |
| Bilirubin (Total) | 0.1-1.2 mg/dL | Elevated in jaundice, liver disease, bile duct obstruction, hemolytic anemia. |
| Ammonia | 10-80 mcg/dL | Elevated in hepatic encephalopathy, liver failure. Treated with lactulose. |
Nursing consideration: Many medications are hepatotoxic. Monitor liver function tests for patients on statins, acetaminophen (especially overdose), methotrexate, and certain antibiotics.
Thyroid Function Tests
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| TSH | 0.4-4.0 mIU/L | Best screening test for thyroid function. Elevated in hypothyroidism. Low in hyperthyroidism. (Inverse relationship with thyroid hormones.) |
| T3 (Triiodothyronine) | 70-190 ng/dL | Active thyroid hormone. Elevated in hyperthyroidism. |
| T4 (Thyroxine) | 4.5-12.5 mcg/dL | Elevated in hyperthyroidism. Low in hypothyroidism. |
Key concept: TSH and T3/T4 have an inverse relationship. In primary hypothyroidism, TSH is high (the pituitary is working harder to stimulate the failing thyroid) while T3/T4 are low.
Cardiac Markers
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| Troponin I | <0.04 ng/mL | Most specific marker for myocardial injury. Elevated in MI. Rises within 3-6 hours, peaks at 12-24 hours. |
| CK-MB | <5% of total CK | Specific to cardiac muscle damage. Rises within 4-6 hours of MI. Useful for detecting reinfarction. |
| BNP (Brain Natriuretic Peptide) | <100 pg/mL | Elevated in heart failure. Higher levels indicate more severe heart failure. Used to differentiate cardiac vs. pulmonary causes of dyspnea. |
| Total Cholesterol | <200 mg/dL (desirable) | Risk factor for cardiovascular disease when elevated. |
| LDL | <100 mg/dL (optimal) | "Bad" cholesterol. Target below 70 for high-risk cardiac patients. |
| HDL | M: >40 mg/dL F: >50 mg/dL |
"Good" cholesterol. Higher is better. Protective against cardiovascular disease. |
| Triglycerides | <150 mg/dL | Elevated with high-fat diet, obesity, diabetes. Risk for pancreatitis when very high (>500). |
Arterial Blood Gases (ABGs)
| Value | Normal Range | Interpretation |
|---|---|---|
| pH | 7.35-7.45 | Below 7.35 = acidosis. Above 7.45 = alkalosis. |
| PaCO2 | 35-45 mmHg | Respiratory component. High = respiratory acidosis. Low = respiratory alkalosis. |
| HCO3- (Bicarbonate) | 22-26 mEq/L | Metabolic component. Low = metabolic acidosis. High = metabolic alkalosis. |
| PaO2 | 80-100 mmHg | Oxygen level in blood. Below 60 = hypoxemia (significant). |
| SaO2 | 95-100% | Oxygen saturation. Below 90% requires intervention. |
How to Interpret ABGs (ROME Method)
Respiratory = Opposite: If pH and CO2 move in opposite directions, it is a respiratory problem.
Metabolic = Equal: If pH and HCO3 move in the same direction, it is a metabolic problem.
- Respiratory Acidosis: pH low, CO2 high. Caused by hypoventilation (COPD, sedation, airway obstruction).
- Respiratory Alkalosis: pH high, CO2 low. Caused by hyperventilation (anxiety, pain, mechanical ventilation).
- Metabolic Acidosis: pH low, HCO3 low. Caused by DKA, renal failure, severe diarrhea, lactic acidosis.
- Metabolic Alkalosis: pH high, HCO3 high. Caused by prolonged vomiting, NG suction, excessive antacid use.
Glucose and Diabetes
| Lab Test | Normal Range | Clinical Significance |
|---|---|---|
| Fasting Blood Glucose | 70-100 mg/dL | 100-125 = prediabetes. 126+ = diabetes. Below 70 = hypoglycemia. |
| HbA1c (Glycated Hemoglobin) | <5.7% (normal) 5.7-6.4% (prediabetes) 6.5%+ (diabetes) |
Reflects average blood glucose over past 2-3 months. Target for most diabetics: below 7%. |
| Random Blood Glucose | <140 mg/dL | 200+ with symptoms is diagnostic for diabetes. |
Critical values: Blood glucose below 54 mg/dL is a medical emergency (severe hypoglycemia). Treat with 15-20g of fast-acting carbohydrate. Above 300 mg/dL, check for DKA (Type 1) or HHS (Type 2). Signs of hypoglycemia: shakiness, sweating, confusion, tachycardia. Signs of hyperglycemia: polyuria, polydipsia, polyphagia, fruity breath (DKA).
Therapeutic Drug Levels
These medications have narrow therapeutic ranges, meaning the difference between a therapeutic dose and a toxic dose is small. The NCLEX frequently tests your knowledge of these levels.
| Medication | Therapeutic Range | Signs of Toxicity |
|---|---|---|
| Digoxin | 0.5-2.0 ng/mL | Nausea, vomiting, yellow-green visual halos, bradycardia, arrhythmias |
| Lithium | 0.6-1.2 mEq/L | Tremors, nausea, diarrhea, confusion, seizures (above 1.5 mEq/L) |
| Phenytoin (Dilantin) | 10-20 mcg/mL | Nystagmus, ataxia, slurred speech, confusion |
| Theophylline | 10-20 mcg/mL | Nausea, vomiting, tachycardia, arrhythmias, seizures |
| Vancomycin (Trough) | 10-20 mcg/mL | Nephrotoxicity, ototoxicity, "Red Man Syndrome" (infuse slowly) |
| Aminoglycosides (Gentamicin Trough) | <2 mcg/mL | Nephrotoxicity (monitor creatinine), ototoxicity (hearing loss, tinnitus) |
Tips for Memorizing Lab Values
- Focus on the most commonly tested values first: Potassium, sodium, hemoglobin, INR/PT, aPTT, blood glucose, and troponin appear most frequently on the NCLEX.
- Learn the clinical significance, not just the number: Knowing that potassium is 3.5-5.0 is less important than knowing that hypokalemia causes cardiac arrhythmias and increases digoxin toxicity risk.
- Use flashcards with spaced repetition: This is the most evidence-based method for memorizing reference values. Review daily during your prep period.
- Group related values together: Study electrolytes as a group, coagulation studies as a group, and so on. Understanding the relationships (e.g., calcium and phosphorus are inverse) makes them easier to remember.
- Practice with application questions: Lab value questions on the NCLEX always include a clinical scenario. Practice identifying abnormal values and choosing the correct nursing action.
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