A Complete Blood Count (CBC) is one of the most common blood tests. It's like a report card for your blood, measuring all the major components. Let's break down what each number means.

Red Blood Cell Tests:

• RBC Count: Number of red blood cells per microliter (normal: 4.2-6.2 million/μL)
• Hemoglobin (Hb): Amount of oxygen-carrying protein (men: 14-18 g/dL; women: 12-16 g/dL)
• Hematocrit (Hct): Percentage of blood volume that's red blood cells (men: 40-54%; women: 36-48%)

White Blood Cell Tests:

• WBC Count: Total white blood cells per microliter (normal: 4,500-11,000/μL)
• Differential: Breakdown of each type (neutrophils, lymphocytes, etc.)

Platelet Tests:

• Platelet count: Number of platelets per microliter (normal: 150,000-400,000/μL)

Why it matters: These numbers help diagnose anemia, infection, leukemia, bleeding disorders, and many other conditions.

Understanding what abnormal values mean is crucial for clinical practice. Let's explore the most common abnormalities:

Anemia (Low Hemoglobin/Hematocrit)

When oxygen-carrying capacity is reduced, patients experience fatigue, weakness, pale skin, and shortness of breath. Causes include:

• Iron deficiency (most common worldwide)
• Vitamin B12 or folate deficiency
• Blood loss (acute or chronic)
• Chronic disease
• Bone marrow disorders

Polycythemia (High Hemoglobin/Hematocrit)

Too many red blood cells makes blood thick and increases clotting risk. Causes include:

• Dehydration (relative increase)
• Chronic hypoxia (living at high altitude, lung disease)
• Polycythemia vera (bone marrow disorder)

Leukocytosis (High WBC)

Usually indicates infection or inflammation. The type of WBC that's elevated provides clues:

• High neutrophils: Bacterial infection
• High lymphocytes: Viral infection
• High eosinophils: Parasites or allergies

Thrombocytopenia (Low Platelets)

Increased bleeding risk — patients may bruise easily or have prolonged bleeding from minor cuts. Causes include bone marrow disorders, certain medications, and autoimmune conditions.

Blood tests also measure electrolytes — charged minerals essential for nerve conduction, muscle contraction, and fluid balance.

Key electrolytes and normal ranges:

• Sodium (Na+): 135-145 mEq/L — controls fluid balance and nerve function
• Potassium (K+): 3.5-5.0 mEq/L — critical for heart rhythm and muscle contraction
• Calcium (Ca2+): 8.5-10.5 mg/dL — bone health, muscle contraction, blood clotting
• Chloride (Cl-): 95-105 mEq/L — works with sodium for fluid balance
• Bicarbonate (HCO3-): 22-26 mEq/L — major buffer for acid-base balance

Why potassium is so important: Even small changes in potassium levels can be dangerous. Hyperkalemia (high potassium) can cause fatal heart arrhythmias. Hypokalemia (low potassium) causes muscle weakness and cardiac problems.

Nursing considerations:

• Verify patient identity before drawing blood
• Explain any fasting requirements
• Use proper technique to avoid hemolysis (which falsely elevates potassium)
• Report critical values immediately