Can Rheumatoid Factor Be Elevated in Psoriatic Arthritis?
Yes, rheumatoid factor (RF) can be elevated in psoriatic arthritis, but this occurs in only a small minority of patients (approximately 3-22% depending on the study), and when present, it raises important diagnostic questions about whether the patient actually has rheumatoid arthritis rather than psoriatic arthritis. 1, 2, 3
Diagnostic Significance of RF in PsA
Rheumatoid factor negativity is a defining characteristic of psoriatic arthritis and is explicitly included in the CASPAR diagnostic criteria as one of the classification features. 4, 1 The fundamental distinction between PsA and RA relies on the absence of RF combined with specific clinical patterns of joint inflammation and the presence of psoriatic skin or nail lesions. 1
When RF is Detected in PsA Patients
The presence of RF in a patient with psoriasis and inflammatory arthritis should prompt careful reconsideration of the diagnosis:
RF positivity occurs in approximately 3% of PsA patients according to general estimates, though some studies report rates up to 22% in certain PsA subtypes. 2, 3
When RF is present in patients with psoriasis and polyarthritis, it may indicate the RA nature of the joint disease rather than true PsA. 2 This is particularly important because some patients may have coincidental RA and psoriasis occurring together rather than PsA.
Patients with RF-positive "PsA" tend to be older and have different joint involvement patterns, with greater involvement of metacarpophalangeal joints, elbow joints, and shoulder joints. 5
Clinical Patterns and RF Positivity
The relationship between RF and PsA varies by disease subtype:
In PsA type 1 (distal interphalangeal joint predominant), RF is essentially never detected. 2
In PsA type 3 (symmetric polyarthritis resembling RA), RF was found in 22% of patients in one study, suggesting diagnostic overlap or misclassification. 2
Some patients initially RF-positive later became RF-negative when sacroiliitis developed, suggesting the initial diagnosis may have been incorrect. 6
Practical Diagnostic Approach
When encountering a patient with psoriasis, inflammatory arthritis, and positive RF:
Consider whether this represents true PsA with incidental RF positivity versus coincidental RA and psoriasis. 2, 6
Evaluate for anti-CCP antibodies, which are more specific for RA. 5 When both RF and anti-CCP are positive in a patient with psoriasis and polyarthritis, RA is more likely than PsA.
Assess the pattern of joint involvement: Symmetric small joint polyarthritis with RF positivity is more consistent with RA, while asymmetric oligoarthritis, DIP involvement, dactylitis, and enthesitis favor PsA even if RF is present. 4, 1, 5
Look for characteristic PsA features that would support the diagnosis despite RF positivity: nail dystrophy (80-90% of PsA patients), dactylitis, enthesitis, and radiographic evidence of new bone formation. 4, 1
Important Caveats
The presence of RF should be considered an exclusion criterion for PsA diagnosis when accompanied by typical RA features, particularly symmetric polyarthritis without characteristic PsA manifestations like dactylitis, enthesitis, or DIP involvement. 2 However, low-level RF positivity can occasionally occur in true PsA and should not automatically exclude the diagnosis if other features strongly support PsA. 3, 5
A common pitfall is failing to recognize that some patients have both conditions simultaneously—true RA occurring in a patient who also has psoriasis—rather than PsA. 2, 6 This distinction has therapeutic implications, as treatment approaches may differ.