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The 'distal-dorsal difference': a thermographic parameter by which to differentiate between primary and secondary
M E Anderson1, T L Moore, M Lunt
1University of Liverpool Adademic Rheumatology Unit, University Hospital Aintree, Liverpool, UK.
Objective:
To test the hypothesis that in a patient with Raynaud's phenomenon (RP), a difference of >1 degrees C between the fingertips and the dorsum of the hand ['distal-dorsal difference' (DDD), fingers cooler] is specific for underlying structural vascular disease as occurs in systemic sclerosis (SSc), and to evaluate other thermographic parameters in the separation of secondary from primary RP.
Methods:
A retrospective analysis of the case notes and thermography results of patients attending thermography, primarily over a 2-yr period. Multinomial logistic regression was used to ascertain whether thermography variables differed between groups with primary RP (56 patients), undifferentiated connective tissue disease (21 patients) and SSc (45 patients), with adjustment for age, sex and smoking.
Results:
A DDD >1 degrees C in any finger at 30 degrees C had a positive predictive value of 70%, and a negative predictive value of 82%, in identifying the patient with RP secondary to SSc. From the results of the multinomial logistic regression, a score was derived incorporating age, number of fingers with DDD >1 degrees C at 30 degrees C and maximum rewarming gradient. This score (with a suitable cut-off) was 82% sensitive and 82% specific in identifying RP secondary to SSc, with a positive predictive value of 73% and a negative predictive value of 89%.
Conclusion:
Parameters derived from thermography (incorporating both a heat and cold challenge) aid in the prediction of SSc in patients with RP.
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