Which method is least likely to provide a definitive result for the diagnosis of rheumatoid arthritis?

The least reliable method for definitively diagnosing rheumatoid arthritis among the provided options is immunofluorescence testing for antinuclear antibodies (ANAs). While ANAs can be present in various autoimmune conditions, including systemic lupus erythematosus and other rheumatological disorders, they are not specific to rheumatoid arthritis. Consequently, a positive ANA test may indicate the presence of other conditions rather than confirming rheumatoid arthritis.

In contrast, the other methods tend to be more relevant for diagnosing rheumatoid arthritis. For example, anti-CCP (cyclic citrullinated peptide) antibodies are known to be highly specific for rheumatoid arthritis and can be detected even in the early stages of the disease, making it a more definitive diagnostic marker. Similarly, agglutination testing for rheumatoid factor can provide supportive evidence, although it is not exclusively indicative of rheumatoid arthritis since rheumatoid factor can be elevated in other conditions as well. Nephelometric measurement of anti-IgG can also provide useful information regarding immune response but is less commonly utilized directly for diagnosing rheumatoid arthritis compared to the others.

Therefore, the presence of ANAs through immunofluorescence testing does not solely correlate with rheumatoid arthritis, making it the least definitive choice for diagnosis in this scenario.