Rheumatoid Arthritis (RA): The patient's joint pain, low white blood cell count (leukopenia), and elevated monocytes could be indicative of RA, especially in the context of an elderly patient. RA can cause a variety of hematologic abnormalities, including leukopenia and thrombocytosis (elevated platelet count), which might be seen as a reactive process.

Felty's Syndrome: This condition is characterized by the combination of rheumatoid arthritis, splenomegaly, and neutropenia (low neutrophil count), which could align with the patient's low WBC count and joint pain. The elevated monocyte count could be part of the disease's inflammatory process.
Chronic Infections or Inflammatory Conditions: Conditions such as endocarditis, osteomyelitis, or chronic inflammatory bowel disease could lead to elevated monocytes and alterations in other blood cell lines due to chronic inflammation and infection.
Medication-Induced Neutropenia: Certain medications can cause neutropenia, and given the patient's age, polypharmacy could be a concern. The joint pain could be unrelated or part of a medication's side effect profile.

Acute Leukemia: Although less likely given the patient's presentation, acute leukemia (such as acute myeloid leukemia) could present with abnormal blood cell counts, including leukopenia, thrombocytosis, and an elevated monocyte count. This diagnosis is critical to not miss due to its severe prognosis if left untreated.
Sepsis: In elderly patients, sepsis can present atypically, and an elevated monocyte count along with leukopenia could be seen in the context of severe infection. Sepsis is a medical emergency that requires prompt recognition and treatment.
Bone Marrow Failure Syndromes: Conditions like aplastic anemia or myelodysplastic syndromes could present with cytopenias (low blood cell counts) and might be suggested by the patient's blood counts, although the elevated platelet count might be less typical.

Histiocytic Disorders: Rare conditions such as Langerhans cell histiocytosis or Erdheim-Chester disease could present with elevated monocytes and systemic symptoms, including joint pain. These diagnoses are less common but should be considered in the differential diagnosis of unexplained cytopenias and monocytosis.
Castleman Disease: A rare disorder that can present with lymphadenopathy, fever, and abnormal blood cell counts, including an elevated monocyte count. It's a diagnosis that might be considered in patients with unexplained multi-system symptoms.