What laboratory test should be ordered first for a patient with chronic kidney disease (CKD) presenting with fatigue, weakness, and occasional shortness of breath?

Laboratory Testing for a CKD Patient with Fatigue, Weakness, and Shortness of Breath

A complete blood count (CBC) should be ordered first for a patient with chronic kidney disease presenting with fatigue, weakness, and occasional shortness of breath, as anemia is the most likely cause of these symptoms in CKD patients. 1

Rationale for CBC as First-Line Test

Anemia is extremely common in CKD patients and directly causes the constellation of symptoms described:

• Fatigue, weakness, and shortness of breath are classic manifestations of anemia in CKD patients 1
• The primary cause is insufficient erythropoietin production by diseased kidneys 1
• Anemia in CKD is typically normocytic and normochromic 1
• The prevalence of anemia increases as kidney function declines, affecting most patients with advanced CKD

What the CBC Will Reveal

A CBC provides critical diagnostic information:

• Hemoglobin level - confirms presence and severity of anemia
• Red blood cell indices (MCV, MCH, MCHC) - helps classify the type of anemia
• Red cell distribution width (RDW) - indicates variability in red cell size
• White blood cell count - rules out infection as a cause of symptoms
• Platelet count - identifies any concurrent thrombocytopenia

Follow-up Testing After CBC

Once anemia is confirmed by CBC, additional tests should be ordered to determine the specific cause and guide treatment:

1. Reticulocyte count - A low count confirms hypoproliferative anemia typical in CKD 1, 2

2. Iron studies - Serum ferritin and transferrin saturation to assess iron status 1

   • Ferritin <100 ng/mL or TSAT <20% indicates absolute iron deficiency
   • Ferritin 100-500 ng/mL with TSAT <30% suggests functional iron deficiency

3. Basic metabolic panel (BMP) - To assess current kidney function and electrolyte status 3

   • Creatinine and BUN to determine CKD severity
   • Electrolytes to identify imbalances common in CKD

Why Other Tests Are Less Appropriate Initially

• BMP alone: While useful for assessing kidney function, it won't identify anemia as the cause of symptoms 3
• Sedimentation rate: Non-specific inflammatory marker, less helpful than direct assessment of anemia
• Ferritin level alone: Important but should follow CBC confirmation of anemia 1
• C-reactive protein: Measures inflammation but doesn't address the most likely cause of symptoms

Clinical Implications and Management

After confirming anemia with CBC:

1. If hemoglobin <10 g/dL with low reticulocyte count and low iron studies:

   • Consider iron supplementation (oral or IV depending on severity)
   • Evaluate need for erythropoiesis-stimulating agents

2. If hemoglobin <10 g/dL with normal iron studies:

   • Consider erythropoiesis-stimulating agents if symptoms are significant
   • Monitor response with serial CBC testing

Common Pitfalls to Avoid

• Failing to distinguish between absolute and functional iron deficiency in CKD patients 1
• Not recognizing that inflammation can affect iron metabolism in CKD
• Overlooking other potential causes of anemia (blood loss, hemolysis)
• Treating anemia without comprehensive evaluation of iron status

By starting with a CBC, you establish the presence and severity of anemia, which is the most likely explanation for this CKD patient's symptoms, and can then proceed with targeted follow-up testing to guide appropriate management.