Rheumatology Advance Access originally published online on June 17, 2008
Rheumatology 2008 47(8):1260; doi:10.1093/rheumatology/ken228
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Comment on: Sparing of the thumb in Raynaud's phenomenon
Southport and Formby District General Hospital – Rheumatology, Southport, UK.
Correspondence to: K. A. Binymin, Southport and Formby District General Hospital – Rheumatology, Kew, Southport, PR8 6PN, UK. E-mail: khalid.binymin{at}southportandormskirk.nhs.uk
SIR, I read with interest the article by Chikura et al. [1]. In their conclusion, they confirm that the thumbs are spared in patients with RP, both primary and secondary, as demonstrated by both symptoms and thermography. A distal–dorsal difference (DDD) in temperature at 23°C of –1°C or less was considered to be clinically relevant. The DDD of the thumb compared with that of the other fingers was used to establish that outcome.
Thermography as a test exhibits considerable temporal variations in the measured values, which are due to both technical factors and physiological characteristics of the blood flow. It often lacks reproducibility and has a wide inter-observer variability [2].
Although DDD specificity and sensitivity have been examined in other fingers affected with RP [3], we do not have database or previous studies that define a cut-off point or normal value nor a reference pattern for thumb DDD that we can use to assert sparing of the thumb with confidence. The study test did not control for finger length, or width being unique characteristics of the thumb that could influence test specificity and sensitivity.
The use of control groups has one major purpose: to allow discrimination of patient outcomes. By using a concurrent group in this study (primary vs secondary RP) and adopting poorly constructed new (thumb) DDD test made it difficult to discriminate the degree of change when the test result is unidirectional (normal thumb DDD).
A three-arm trial including an active control as well as a normal-control group can readily assess whether a failure to distinguish the test group from normal controls implies sparing was due to non-involvement by the pathological process or is simply the result of a trial test that lacked the ability to discriminate between a lower degree of involvement.
The comparison of normal with RP groups in such a trial provides evidence of test sensitivity. It is possible to make the active groups larger than the normal group in order to improve the precision of the test comparison, if this is considered important.
I find it difficult to accept the conclusion statement that the thumbs are spared based on the test method adopted in this study.
Disclosure statement: The author has declared no conflicts of interest.
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- Chikura B, Moore TL, Manning JB, Vail A, Herrick AL. Sparing of the thumb in Raynaud's phenomenon. Rheumatology (2008) 47:219–21.
[Abstract/Free Full Text] - Zaproudina N, Varmavuo V, Airaksinen O, Närhi M. Reproducibility of infrared thermography measurements in healthy individuals. Physiol Meas (2008) 29:515–24.[CrossRef][Web of Science][Medline]
- Anderson ME, Moore TL, Lunt M, Herrick AL. The ‘distal-dorsal difference’: a thermographic parameter by which to differentiate between primary and secondary Raynaud's phenomenon. Rheumatology (2007) 46:533–8.
[Abstract/Free Full Text]
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