For a patient with an elevated white blood cell count during fever and joint pain, what is the first step in diagnosis?

In a clinical scenario where a patient presents with fever and joint pain accompanied by an elevated white blood cell count, the first step in diagnosis is typically a joint aspiration. This procedure is critical as it allows for the collection of synovial fluid from the affected joint, which can be directly analyzed for the presence of infection, crystals, or other joint pathologies.

Analyzing the synovial fluid provides valuable information that can assist in distinguishing between various types of arthritis or joint disorders, including septic arthritis, gout, and rheumatoid arthritis. The cytology of the fluid can help determine the white blood cell count, specifically whether there is a predominance of neutrophils suggestive of infection, or other cell types indicative of different inflammatory processes.

In contrast, while a complete blood count can provide general information about the patient's hematologic status and confirm leukocytosis, it lacks the specificity needed to pinpoint the cause of the joint symptoms. Similarly, serum ferritin measurement can offer insights into the inflammatory process or possible anemia but does not directly address the joint pathology. An X-ray of the joints can help visualize structural abnormalities but would not provide immediate information about any infectious or inflammatory processes ongoing in the joint itself. Thus, joint aspiration is the most logical first step