Objectives: To investigate the influence of a standardised cold stress test (CST) on the thermographic 'distal-dorsal difference' (DDD) and its capacity to differentiate between disease states in the assessment of Raynaud's phenomenon (RP), and to compare the discriminatory capacity of the DDD of individual digits with composite indices of multiple digits.
Methods: Thermographic images of 55 patients with primary RP (PRP, n = 27) and systemic sclerosis (SSc, n=28) who had undergone assessment of RP were retrospectively reviewed. The DDD for individual digits, and composite scores of multiple digits, were calculated at baseline (23 degrees C), and at 10 min following CST. The discriminatory capacity of the mean DDD, and the proportion of patients with a clinically meaningful DDD of <-1 degrees C, were assessed for individual digits and composite indices, at baseline and following cold challenge.
Results: There was a more pronounced decrease of the DDD (indicating reduced distal perfusion) following CST in patients with PRP compared to SSc. The disparity in response to CST between groups narrowed the differences that were present at baseline, reducing the discriminatory capacity of the DDD for all endpoints. Sparing of the thumbs occurs to a greater extent in SSc (P<0.005) compared with PRP (P<0.05) but does not facilitate differentiation between groups. Large variability of the DDD within groups precludes easy differentiation between disease states. Composite indices of multiple digits are preferable to individual digital assessment.
Conclusions: The discriminatory capacity of the DDD is lost following CST. The CST may not be essential in the thermographic assessment of RP, potentially allowing greater use of thermography in clinical practise.