Which investigation should be ordered if there is suspicion of rheumatoid arthritis?

In the context of suspected rheumatoid arthritis, ordering a platelet count can provide valuable diagnostic information. Rheumatoid arthritis is associated with inflammation, which often leads to changes in blood cell counts, including an increase in platelets. This condition, known as thrombocytosis, may occur as part of the body's response to inflammation.

In addition to platelets, other blood tests that are often used in conjunction include rheumatoid factor and anti-citrullinated protein antibody testing. These tests assess the presence of specific antibodies that are commonly found in individuals with rheumatoid arthritis. However, platelets specifically can serve as an initial investigation to indicate an inflammatory process, thus guiding further diagnostic steps.

Magnetic resonance imaging and CT scans are more specialized imaging techniques that are typically not the first-line investigations in diagnosing rheumatoid arthritis but may be used later to assess joint damage or complications. An echocardiogram is used primarily to evaluate cardiac conditions and is not relevant for diagnosing rheumatoid arthritis. Therefore, the most appropriate investigation in this scenario, aimed at identifying signs of inflammation that could suggest rheumatoid arthritis, is a platelet count.