When do you order a Complete Blood Count (CBC) versus or in addition to a Basic Metabolic Panel (BMP)?

When to Order CBC vs. BMP in Clinical Practice

A CBC should be ordered for patients with suspected hematologic disorders, infections, or inflammatory conditions, while a BMP should be ordered for patients with suspected metabolic, renal, or electrolyte abnormalities. Both tests should be ordered together when evaluating patients with systemic illness, preoperative assessment, or monitoring medication effects.

CBC Indications

A Complete Blood Count (CBC) provides critical information about blood cell components and should be ordered in the following clinical scenarios:

• Suspected hematologic disorders:

  • Anemia (fatigue, pallor, shortness of breath)
  • Bleeding disorders (easy bruising, prolonged bleeding)
  • Leukemia or other hematologic malignancies

• Infection evaluation:

  • Fever of unknown origin
  • Suspected bacterial infections (elevated WBC with neutrophilia)
  • Viral infections (lymphocytosis)
  • Sepsis evaluation 1

• Inflammatory conditions:

  • Autoimmune disorders
  • Chronic inflammatory diseases
  • Monitoring inflammation severity

• Medication monitoring:

  • Immunosuppressive agents (azathioprine, methotrexate) 2
  • Medications with known hematologic side effects

BMP Indications

A Basic Metabolic Panel (BMP) assesses electrolytes, kidney function, and glucose levels and should be ordered in these scenarios:

• Electrolyte abnormalities:

  • Dehydration
  • Vomiting or diarrhea
  • Heart failure
  • Medication effects (diuretics)

• Kidney function assessment:

  • Suspected renal disease
  • Monitoring chronic kidney disease
  • Before administering nephrotoxic medications 3

• Metabolic disorders:

  • Diabetes management
  • Acid-base disturbances
  • Malnutrition evaluation

• Medication monitoring:

  • Drugs requiring renal dosing
  • Medications affecting electrolytes

When to Order Both CBC and BMP

Both tests should be ordered together in these clinical scenarios:

• Initial evaluation of systemic illness:

  • Patients with HIV upon initiation of care 2
  • Suspected sepsis or severe infection 2
  • Unexplained weight loss or fatigue

• Preoperative assessment:

  • Based on surgical risk and patient factors 2
  • For cardiovascular surgery
  • For patients with comorbidities (ASA class 2 or 3)
  • For elderly patients undergoing major surgery

• Medication initiation and monitoring:

  • Before starting medications with potential hematologic and metabolic effects
  • Monitoring for systemic immunosuppressive therapies 2
  • Before initiating nephrotoxic medications 2

• Chronic disease management:

  • Diabetes with complications
  • Heart failure
  • Chronic kidney disease
  • Liver disease

Special Considerations

• Age-specific considerations:

  • Elderly patients often require both tests due to higher prevalence of comorbidities
  • Pediatric patients may need age-appropriate reference ranges

• Cost-effectiveness considerations:

  • Avoid routine CBC ordering without specific indications 4
  • Consider targeted testing based on clinical presentation

• Disease-specific protocols:

  • For psoriasis treatment monitoring, regular CBC is recommended 2
  • For HIV patients, both CBC and chemistry panel are recommended at initiation of care 2

Common Pitfalls to Avoid

1. Ordering CBC routinely for all patients without specific indications
2. Failing to order BMP when evaluating patients on medications affecting electrolytes
3. Not considering both tests when evaluating systemic illness
4. Overlooking the need for baseline values before starting medications with potential hematologic or metabolic effects

Remember that while these tests provide valuable diagnostic information, they should be interpreted within the clinical context and in conjunction with a thorough history and physical examination 5.