Diagnostic tests and lab values
Introduction
Lab tests are like the body’s report card; they reveal infection, organ failure, acid-base imbalances, and more. As a nurse, you don’t just document the numbers, you interpret them, recognize red flags, and act fast when something’s off. NCLEX questions frequently ask you to evaluate critical lab values and choose the safest next step.
This section reviews commonly tested laboratory values, their normal reference ranges, clinical significance, and appropriate nursing interventions.
Learning objectives
By the end of this section, you should be able to:
- Identify normal reference ranges for CBC, BMP, LFTs, and ABGs
- Interpret lab abnormalities and link them to common disease states
- Recognize critical values that require immediate action
- Apply nursing interventions for abnormal diagnostic findings
Complete blood count (CBC)
| Component | Normal Range | Meaning | |
| WBC | 4,500–11,000 /mm³ | ↑ = infection, ↓ = leukopenia; increased infection risk | |
| Hemoglobin (Hgb) | 12–16 g/dL (female) | ↓ = anemia or bleeding | |
| 14–18 g/dL (male) | ↓ = anemia or bleeding | ||
| Hematocrit (Hct) | 37–47% (female) | Mirrors Hgb changes | |
| 42–52% (male) | Mirrors Hgb changes | ||
| Platelets | 150,000–400,000 /mm³ | ↓ = bleeding risk, ↑ = clotting risk |
Basic metabolic panel (BMP)
| Electrolyte | Normal Range | Clinical Significance |
| Sodium (Na⁺) | 135–145 mEq/L | ↓ = fluid overload or sodium loss, ↑ = dehydration |
| Potassium (K⁺) | 3.5–5.0 mEq/L | ↓ or ↑ = cardiac arrhythmias |
| Calcium (Ca²⁺) | 8.5–10.5 mg/dL | ↓ = paresthesias, tetany, muscle cramps |
| Magnesium (Mg²⁺) | 1.5–2.5 mEq/L | ↓ = ↑ reflexes, seizures; ↑ = ↓ reflexes |
| BUN | 10–20 mg/dL | ↑ = kidney dysfunction or dehydration |
| Creatinine | 0.6–1.3 mg/dL | ↑ = impaired kidney function |
| Glucose (fasting) | 70–99 mg/dL | ↑ = hyperglycemia, ↓ = hypoglycemia |
| CO₂ (bicarbonate) | 22–28 mEq/L | Acid-base balance (paired with ABG) |
Arterial blood gases (ABGs)
| Value | Normal Range | Significance |
| pH | 7.35–7.45 | ↓ = acidosis, ↑ = alkalosis |
| PaCO₂ | 35–45 mmHg | Respiratory indicator: ↓ = alkalosis, ↑ = acidosis |
| HCO₃⁻ (bicarb) | 22–26 mEq/L | Metabolic indicator: ↓ = acidosis, ↑ = alkalosis |
| PaO₂ | 80–100 mmHg | Oxygenation status |
| SaO₂ | >95% | Hemoglobin saturation with oxygen |
Quick interpretation:
- ↓ pH + ↑ PaCO₂ = Respiratory acidosis (e.g., COPD exacerbation, sedative overdose)
- ↓ pH + ↓ HCO₃⁻ = Metabolic acidosis (e.g., diabetic ketoacidosis, renal failure)
- ↑ pH + ↓ PaCO₂ = Respiratory alkalosis (e.g., anxiety, hyperventilation)
- ↑ pH + ↑ HCO₃⁻ = Metabolic alkalosis (e.g., prolonged vomiting, diuretic therapy)
- //////Caption: Stepwise Interpretation of Arterial Blood Gases
- Illustration type: Decision flowchart
- Illustration note: Check pH → Acidosis or alkalosis? → Check PaCO₂ → Respiratory? → Check HCO₃⁻ → Metabolic? → Assess compensation///////
Liver function tests (LFTs)
| Test | Normal Range | Clinical Significance |
| AST/ALT | 10–40 IU/L | Liver damage (e.g., hepatitis, alcohol use) |
| Bilirubin (total) | 0.1–1.2 mg/dL | Jaundice, bile duct obstruction, liver dysfunction |
| Albumin | 3.5–5.0 g/dL | Low in liver disease, malnutrition, or protein loss |
| Ammonia | 15–45 mcg/dL | ↑ in hepatic encephalopathy |
| PT/INR | PT: 11–13 sec | ↑ with liver disease, warfarin therapy |
| INR: 0.8–1.1 (normal) | Therapeutic range for most warfarin indications: 2.0–3.0 | |
| aPTT | 25-45 sec | ↑ with liver disease, heparin therapy |
Critical values: Know when to act
| Lab | Critical value | Nursing action |
| Potassium | <2.5 or >6.5 mEq/L | Place on cardiac monitor and notify the provider |
| Sodium | <120 or >160 mEq/L | Risk of seizures or coma. Implement seizure precautions as indicated and notify the provider |
| Glucose | <50 or >400 mg/dL | Hypoglycemia = administer glucose or dextrose; Hyperglycemia = administer insulin as ordered |
| INR | >4 | Increased bleeding risk; notify the provider and anticipate intervention |
| WBC | <2,000 or >30,000/mm³ | Assess for signs of infection, implement neutropenic precautions as indicated, and notify the provider |
| Platelets | <50,000 /mm³ | Risk of spontaneous bleeding; institute bleeding precautions and notify the provider |
| pH (ABG) | <7.25 or >7.55 | Indicates a critical acid-base disturbance; assess respiratory status, support ABCs, and notify the provider |
Clinical vignette: A 64-year-old client with pneumonia has the following ABG results:
- pH 7.28
- PaCO₂ 52 mmHg
- HCO₃⁻ 25 mEq/L
Nursing action: The nurse identifies respiratory acidosis, assesses the client’s respiratory status, administers oxygen as prescribed, encourages deep breathing, and notifies the provider.
Rationale: The low pH with an elevated PaCO₂ and normal HCO₃⁻ indicates uncompensated respiratory acidosis, likely caused by hypoventilation associated with pneumonia.