Differential Diagnosis for an 80-year-old Woman with Increasing Fatigue

Single Most Likely Diagnosis

• Anemia of chronic disease: This diagnosis is most likely due to the patient's history of systemic lupus erythematosus (SLE), a chronic inflammatory condition. The laboratory findings of low iron levels (45 mcg/dL) despite elevated ferritin levels (180 ng/mL) are consistent with anemia of chronic disease, where iron is sequestered and unavailable for erythropoiesis due to chronic inflammation.

Other Likely Diagnoses

• Iron deficiency anemia: Although the ferritin level is elevated, which might suggest that iron stores are adequate, the low iron level and the clinical context could still suggest iron deficiency anemia, especially if there's an underlying chronic blood loss that is not immediately apparent (e.g., gastrointestinal bleeding not detected by fecal occult blood test).
• Myelodysplastic syndrome: Given the patient's age and the presence of anemia with a relatively low reticulocyte count, myelodysplastic syndrome (a clonal disorder of hematopoietic stem cells) could be considered, although it would typically require bone marrow examination for diagnosis.

Do Not Miss Diagnoses

• Hemolysis: Although less likely given the relatively low lactate dehydrogenase (LDH) and indirect bilirubin, hemolysis (especially if related to SLE) could be a critical diagnosis not to miss. The presence of an elevated erythrocyte sedimentation rate (ESR) and a history of SLE might suggest an autoimmune hemolytic anemia, which would require further investigation (e.g., direct Coombs test).
• Acute blood loss: Despite the negative fecal occult blood test, occult blood loss could still be a consideration, especially if the patient has a source of bleeding that is not being detected by standard tests. Acute blood loss would typically present with a more rapid onset of symptoms and possibly more significant changes in hemoglobin levels over time.

Rare Diagnoses

• Other autoimmune cytopenias: In the context of SLE, other autoimmune cytopenias (e.g., autoimmune thrombocytopenia, leukopenia) could be considered, although the platelet and leukocyte counts are within normal limits in this case.
• Paroxysmal nocturnal hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, bone marrow failure, and the potential for blood clot formation. It would be unusual but could be considered in a differential diagnosis for anemia, especially with a background of an autoimmune disease like SLE, though specific tests (e.g., flow cytometry for CD55 and CD59) would be needed for diagnosis.