FBC; ESR; CRP; ANA; RF; anti-cyclic citrullinated peptide; HLA-B27; Syphilis and Tuberculosis testing (e.g., VDRL or RPR, interferon-gamma release assays or tuberculin skin test); Lyme disease testing; Imaging studies, such as chest X-rays or MRI scans. Here, CRP is linked to syphilis.