Differential Diagnosis for 84-year-old Woman with Low Sodium, Low Chloride, and High BUN

Single Most Likely Diagnosis

• Dehydration: This is the most likely diagnosis given the high BUN, which indicates dehydration. The low sodium and chloride levels can be seen in dehydration due to the loss of these electrolytes. In elderly patients, dehydration can occur due to various reasons such as decreased fluid intake, increased fluid loss, or a combination of both.

Other Likely Diagnoses

• Congestive Heart Failure (CHF): CHF can lead to decreased renal perfusion, resulting in elevated BUN. Low sodium and chloride levels can be seen in CHF due to the use of diuretics or due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH) which can be associated with CHF.
• Nephrotic Syndrome: This condition is characterized by significant proteinuria, leading to hypoalbuminemia, which can cause low sodium and chloride levels due to decreased oncotic pressure. The high BUN can be seen due to decreased renal function.
• Gastrointestinal Loss: Significant gastrointestinal loss due to vomiting, diarrhea, or bowel obstruction can lead to dehydration, low sodium, and low chloride levels. The high BUN would support this diagnosis.

Do Not Miss Diagnoses

• Adrenal Insufficiency: Although less common, adrenal insufficiency can present with hyponatremia (low sodium), hypochloremia (low chloride), and elevated BUN due to decreased cortisol and aldosterone production. This diagnosis is critical to consider because it can be life-threatening if not promptly treated.
• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): SIADH can cause hyponatremia and hypochloremia. The elevated BUN might be seen due to dehydration or due to the underlying cause of SIADH, such as malignancy or CNS disorders.

Rare Diagnoses

• Renal Tubular Acidosis (RTA): Certain types of RTA can present with hypokalemia, but some forms can also have hyponatremia and hypochloremia. The high BUN could be seen in the context of renal dysfunction.
• Pseudohypoaldosteronism: A rare condition characterized by resistance to aldosterone, leading to hyponatremia, hypochloremia, and hyperkalemia. The elevated BUN could be due to dehydration or renal impairment.