Rheumatology Advance Access originally published online on April 4, 2006
Rheumatology 2006 45(6):780-781; doi:10.1093/rheumatology/kel084
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
LETTER TO THE EDITOR |
Positive and negative predictive values from published studies can be misleading for decision-making in clinical practice: reply
1 Department of Internal Medicine, Innsbruck Medical University, Innsbruck and 2 General Hospital of the Elizabethenians, Klagenfurt, Austria
Correspondence to: M. Schirmer, Department of Internal Medicine, General Hospital of the Elizabethenians, Völkermarkterstrasse 15-19, A-9020 Klagenfurt, Austria. E-mail: michael.schirmer{at}ekh.at
SIR, The letter to the editor by Rudwaleit and Sieper points out the important role of the likelihood ratio (LR) as a description of the diagnostic value of any diagnostic test in comparison to the restricted usefulness of positive and negative predictive values. The authors demonstrate in various examples the dependence of the positive and negative predictive values on the prevalence of the disease within the population tested. We totally agree with the conclusion of Rudwaleit and Sieper that highlighting the positive predictive value of specific antibodies cross-reacting with a 28 kDa Drosophila antigen in ankylosing spondylitis (AS) patients [1] may be misleading for clinical decision-making.
We also share the opinion that testing patients suspicious for AS for the presence of antibodies cross-reacting with the 28 kDa Drosophila antigen may have an additional diagnostic value. The positive LRs, provided in Table 2, were calculated to be 1.9, 2.2 and 3.8 for the cut-off levels of 
50, 60 and 75 U/ml, respectively. Choosing a higher cut-off of 125 U/ml results in a further increase in the positive LR (7.3, Du et al., unpublished) but decreases the sensitivity of this test below 20%. Then the question arises whether such a gain in the positive LR, paralleled with poor sensitivity, is helpful in clinical practice. Using a cut-off of 75 U/ml, the positive LR of 3.8 is comparable with the diagnostic value of inflammatory back pain, enthesitis of the heel or peripheral arthritis [2], with a sensitivity of 30.7%. [1] As serological signs of inflammation measured by the ESR and CRP levels are often absent in AS patients [3] and diagnosis of AS in HLA-B27 negative patients is unacceptably delayed, [4] a positive enzyme-linked immunosorbent assay (ELISA) test result can contribute to the diagnostic evaluation of these patients, expanding the diagnostic laboratory assessments available so far. Notably, in this study the presence of antibodies cross-reacting with the 28 kDa Drosophila antigen was independent of the HLA-B27 status in AS patients.
For testing the diagnostic values of these specific 28 kDa Drosophila antigen cross-reacting antibodies, only sera from AS patients with a definite diagnosis according to the modified New York criteria were included. As data concerning the diagnostic usefulness of this established ELISA based on the 28 kDa antigen were sparse [5, 6], we intended to perform the study with definitive settings. But we agree with Rudwaleit and Sieper that it is necessary now to prospectively address the diagnostic value of this ELISA test in early AS and other spondylarthropathies.
The authors have declared no conflicts of interest.
References
- Duftner C, Dejaco C, Klauser A, Falkenbach A, Lakomek HJ, Schirmer M. High positive predictive value of specific antibodies cross-reacting with a 28 kDa Drosophila antigen for diagnosis of ankylosing spondylitis. Rheumatology 2006;45:38–42.
[Abstract/Free Full Text] - Rudwaleit M, van der Heijde D, Khan MA, Braun J, Sieper J. How to diagnose axial spondyloarthritis early. Ann Rheum Dis 2004;63:535–43.
[Abstract/Free Full Text] - Tutuncu ZN, Bilgie A, Kennedy LG, Calin A. Interleukin-6, acute phase reactants and clinical status in ankylosing spondylitis. Ann Rheum Dis 1994;53:425–6.[Medline]
- Feldtkeller E, Khan MA, van der Heijde D, van der Linden S, Braun J. Age at disease onset and diagnosis delay in HLA-B27 negative vs. positive patients with ankylosing spondylitis. Rheumatol Int 2003;23:61–6.[Web of Science][Medline]
- Lakomek HJ, Hagemann O, Schochau M, Northemann W. Ein neuer Antikörper für die Frühdiagnose der Spondarthritiden. Akt Rheumatol 1997;22:47–54.
- Wachter U, Lakomek HJ, Pentrop S, Ganser S. Spondylarthropathy-antibody—a predictive parameter to assess courses of disease in juvenile arthritis?. Ann Rheum Dis 2002;61:313–4.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||