What investigation is most useful for differentiating causes of acute monoarthropathy?

Joint aspiration, also known as arthrocentesis, is the most useful investigation for differentiating the causes of acute monoarthropathy. This procedure involves the extraction of synovial fluid from the affected joint and is invaluable for both diagnostic and therapeutic purposes.

By analyzing the synovial fluid, clinicians can determine the underlying cause of the joint swelling and pain. For example, the fluid can be assessed for clarity and color, as clear fluid is often associated with non-inflammatory conditions, while cloudy or turbid fluid can indicate inflammatory conditions such as gout, pseudogout, or septic arthritis. Additionally, fluid analysis can include cell counts, which helps differentiate between non-inflammatory, inflammatory, and septic processes. The presence of crystals can indicate gout or pseudogout, while bacterial cultures can confirm or rule out an infection.

Other methods, such as imaging like X-rays or MRI, primarily adjunct to diagnosis and do not provide the same level of direct diagnostic information regarding the fluid characteristics. Blood tests can be useful for systemic clues but lack specificity for joint conditions on their own. Therefore, joint aspiration remains the definitive first step in distinguishing the etiology of acute monoarthropathy.