Differential Diagnosis for Persistent Macrocytosis

Single Most Likely Diagnosis

• Vitamin B12-related effect: Although the patient's B12 level is >2000, which is significantly elevated, the use of high doses of vitamin B12 can sometimes lead to an imbalance in the utilization of folate and B12, potentially causing macrocytosis. However, the more likely explanation related to B12 in this context is the medication itself, as carbidopa-levodopa can interfere with B12 metabolism, but the direct link to macrocytosis is less clear without considering other factors like the patient's renal function and potential for folate deficiency.

Other Likely Diagnoses

• Chronic Kidney Disease (CKD): With a GFR of 45 and creatinine of 1.20, the patient has impaired renal function. CKD can lead to macrocytosis due to the buildup of toxins that affect erythropoiesis and due to potential deficiencies in erythropoietin, which can affect red blood cell production.
• Folate deficiency: Despite the high B12 levels, a deficiency in folate can cause macrocytosis. The patient's diet, gastrointestinal absorption, and certain medications can affect folate levels.
• Medication effect (other than B12): Certain medications can cause macrocytosis. While carbidopa-levodopa is not commonly associated with this condition directly, other medications the patient might be taking could be contributing factors.

Do Not Miss Diagnoses

• Alcohol abuse: Although not directly indicated in the scenario, alcohol abuse is a common cause of macrocytosis and can have severe health implications if not addressed. It's essential to inquire about alcohol use in patients with unexplained macrocytosis.
• Hypothyroidism: Untreated hypothyroidism can lead to macrocytosis. Given the patient's age, hypothyroidism is a possibility that should not be overlooked.

Rare Diagnoses

• Myelodysplastic syndromes (MDS): These are a group of disorders caused by poorly formed or dysfunctional blood cells, which can lead to macrocytosis among other cytopenias. MDS is less common but should be considered in the differential diagnosis, especially in older adults.
• Autoimmune hemolytic anemia: Although more commonly associated with normocytic or microcytic anemia, some forms can present with macrocytosis due to the reticulocytosis that occurs in response to hemolysis.
• Pernicious anemia: An autoimmune condition leading to B12 deficiency, which can cause macrocytosis. However, the patient's high B12 level makes this less likely unless there's a problem with B12 utilization at the cellular level.