Laboratory Values

CBC, BMP, CMP, coagulation studies, cultures, and critical lab values for respiratory care.

beginner55 min estimated3 lessons

Complete Blood Count (CBC)

The CBC provides critical information about oxygen-carrying capacity, infection, and coagulation.

Key values: - Hemoglobin (Hgb): M 13.5–17.5 g/dL, F 12–15.5 g/dL. Low = anemia → reduced O2 carrying capacity → increased work of breathing - Hematocrit (Hct): M 41–53%, F 36–46%. Rule of 3: Hct ≈ Hgb × 3 - WBC: 4,500–11,000/μL. Elevated = infection/inflammation; neutropenia <1,500 = high infection risk - Platelets: 150,000–400,000/μL. <50,000 = bleeding risk; <20,000 = spontaneous bleeding - Bands/neutrophils: left shift (increased bands) = acute bacterial infection - Polycythemia: Hgb >17 (M) or >15 (F) — seen in chronic hypoxia (COPD), sleep apnea

Anemia reduces O2 carrying capacity — may worsen dyspnea regardless of lung function
WBC >11,000 = leukocytosis (infection/inflammation); <4,500 = leukopenia (viral, immunosuppressed)
Left shift (bands >10%) = acute bacterial infection
Polycythemia in COPD: chronic hypoxia stimulates EPO → more RBCs
Platelet <50,000: avoid invasive procedures; <20,000: transfuse platelets

Clinical tip

In a patient with Hgb of 7 g/dL and respiratory distress, adding supplemental O2 helps but treating the anemia (transfusion) will dramatically improve O2 delivery.

Electrolytes and Renal Function

The Basic Metabolic Panel (BMP) includes sodium, potassium, chloride, bicarbonate, BUN, creatinine, and glucose.

Respiratory implications: - Potassium (K+): normal 3.5–5.0 mEq/L. Hypokalemia (<3.5) causes respiratory muscle weakness and arrhythmias; worsened by albuterol (shifts K+ intracellularly). Hyperkalemia (>5.5) — peaked T waves, arrhythmias - Sodium (Na+): normal 135–145 mEq/L. Hyponatremia <120 can cause respiratory depression - Bicarbonate (HCO3): normal 22–26 mEq/L — reflects metabolic component of acid-base - Creatinine: normal 0.7–1.3 mg/dL — indicates renal function; elevated = AKI, reduced drug clearance - Magnesium: 1.7–2.2 mg/dL; hypomagnesemia causes bronchospasm and arrhythmias

Hypokalemia: arrhythmias + respiratory muscle weakness; worsened by albuterol, diuretics
Hyperkalemia: peaked T-waves, wide QRS, cardiac arrest — ECG before treatment decision
Low HCO3 (<22) = metabolic acidosis; high HCO3 (>26) = metabolic alkalosis or chronic hypercapnia
Magnesium deficiency: bronchospasm, arrhythmias — replete in status asthmaticus
Phosphate depletion: respiratory muscle fatigue, difficult weaning from ventilator

Clinical tip

Check phosphate levels in ICU patients who are failing ventilator weaning — severe hypophosphatemia causes diaphragm weakness and makes extubation impossible.

Infection Markers and Cultures

Identifying infection is critical in respiratory care. Key markers:

- WBC with differential: leukocytosis + left shift = bacterial; lymphocytosis = viral - Procalcitonin (PCT): bacterial infections → elevated (>0.5 μg/L suggests bacterial; >2 = likely); viral/non-infectious → low. Used to guide antibiotic therapy duration - CRP (C-reactive protein): nonspecific inflammatory marker; useful for trending - Lactate: >2 mmol/L = hypoperfusion (sepsis screen); >4 = severe sepsis - Cultures: blood cultures ×2 before antibiotics; sputum culture (>25 PMNs, <10 squamous cells = adequate sample); BAL ≥10⁴ CFU/mL = VAP - Urine Legionella antigen: for atypical pneumonia - BNP/NT-proBNP: elevated in heart failure → differentiates cardiac vs pulmonary edema

PCT: helps distinguish bacterial vs viral; guides antibiotic stewardship
Lactate >4 mmol/L = septic shock; trend lactate to monitor treatment response
Blood cultures ×2 before antibiotics — but don't delay antibiotics >45 min for cultures
Adequate sputum: >25 PMNs and <10 squamous epithelial cells per LPF
BNP >500 pg/mL highly suggests heart failure; <100 makes it unlikely

Clinical tip

In a patient with bilateral infiltrates and hypoxemia, a BNP level can help differentiate cardiogenic pulmonary edema (high BNP) from ARDS/pneumonia (normal or mildly elevated BNP).

Clinical cases

Failing to Wean from Ventilator

A post-op patient on day 5 of mechanical ventilation fails repeated SBTs. She is awake, cooperative, FiO2 0.35, PEEP 5. Recent lab: Phosphate 1.0 mg/dL (normal 2.5–4.5).

What lab abnormality is contributing to ventilator dependence and how should it be treated?

Hypophosphatemia causing respiratory muscle weakness. Treat with IV phosphate replacement.

Phosphate is essential for ATP production in muscle cells including the diaphragm. Severe hypophosphatemia (<1.0 mg/dL) causes profound respiratory muscle weakness, making SBT impossible. Replace IV phosphate (sodium or potassium phosphate) and recheck before next SBT attempt.

NBRC pearls

Albuterol causes hypokalemia — NBRC tests this frequently
Hypophosphatemia causes respiratory muscle weakness and ventilator weaning failure
Procalcitonin guides antibiotic duration in CAP/VAP
BNP differentiates cardiogenic vs non-cardiogenic edema

Memory tricks

Albuterol: 'SABA Shifts Serum K+ down' — moves potassium into cells
Adequate sputum: 'More PMNs, fewer squamies' — >25 PMNs, <10 squamous

Common mistakes

Overlooking hypokalemia caused by repeated albuterol treatments
Not checking phosphate in a patient failing ventilator weaning
Sending inadequate sputum samples (too many squamous cells = oral contamination)
Delaying antibiotics >1 hour waiting for cultures to result

Laboratory Values

Complete Blood Count (CBC)

Electrolytes and Renal Function

Infection Markers and Cultures

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Laboratory Values

Complete Blood Count (CBC)

Electrolytes and Renal Function

Infection Markers and Cultures

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Complete Blood Count (CBC)

Electrolytes and Renal Function

Infection Markers and Cultures

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Complete Blood Count (CBC)

Electrolytes and Renal Function

Infection Markers and Cultures

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics