Increased mean corpuscular hemoglobin concentration

RATIONALE: Mean corpuscular hemoglobin concentration (MCHC) is a stable parameter, and its true elevation typically indicates hematological disorders. Although occasional reports suggest an association with electrolyte imbalances, the underlying mechanisms remain unclear. PATIENT CONCERNS: An 81-year-old female was admitted with a 3-day history of altered mental status. Complete blood count revealed a markedly elevated MCHC (369 g/L), and peripheral blood smear showed reduced central pallor and mildly hyperchromic erythrocytes, suggestive of cellular dehydration. DIAGNOSES: Chronic severe hypotonic hyponatremia (serum sodium 102 mmol/L), initially alerted by a confirmed "true" MCHC elevation after systematically ruling out common analytical interferences. INTERVENTIONS: Laboratory personnel issued an early warning for severe electrolyte imbalance prior to biochemical results. The patient subsequently received targeted sodium replacement therapy. OUTCOMES: Following sodium replacement, the patient's serum sodium levels normalized (142 mmol/L), her mental status markedly improved, and her MCHC demonstrated a mirroring downward trend back to normal (337 g/L). LESSONS: Laboratory physicians should consider true MCHC elevation, after excluding interferences, as a potential indicator of chronic hyponatremia. Proactive communication of this finding to clinicians can expedite diagnosis and treatment of critical electrolyte disorders.