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Initially identified in 1978, spectrum bias refers to the phenomenon that the performance of a diagnostic test may change between different clinical settings owing to changes in the patient case-mix thereby affecting the transferability of study results in clinical practice. In the statistical sense it is not truly a bias and has led some authors to refer to the effect(s) as 'spectrum effects', whilst others maintain it is bias if the true performance of the test differs from that which is 'expected'. Usually the performance of a diagnostic test is measured in terms of its sensitivity and specificity and it is changes in these that are considered when referring to spectrum bias. However, other performance measures such as the likelihood ratios may also be affected by spectrum bias.
Generally it is considered to have three causes. The first is due to a change in the case-mix of those patients with the target disorder (disease) and this affects the sensitivity. The second is due to a change in the case-mix of those without the target disorder (disease-free) and this affects the specificity. The third is due to a change in the prevalence, and this affects both the sensitivity and specificity. This final cause is not widely appreciated, but there is mounting empirical evidence as well as theoretical which suggest that it does indeed affect a test's performance.
Diagnostic test performances reported by some studies may be artificially overestimated if it is a case-control design where a healthy population ('fittest of the fit') is compared with a population with advanced disease ('sickest of the sick') .
See also 
- Ransohoff DF, Feinstein AR (1978). "Problems of spectrum and bias in evaluating the efficacy of diagnostic tests". N. Engl. J. Med. 299 (17): 926–30. doi:10.1056/NEJM197810262991705. PMID 692598.
- Willis BH, (2008). "Spectrum bias—why clinicians need to be cautious when applying diagnostic test studies,". Family Practice. 25: 390-396. (5): 390–6. doi:10.1093/fampra/cmn051. PMID 18765409.
- Mulherin SA, Miller WC (2002). "Spectrum bias or spectrum effect? Subgroup variation in diagnostic test evaluation". Ann. Intern. Med. 137 (7): 598–602. PMID 12353947.
- Willis BH, (2012). "Evidence that disease prevalence may affect the performance of diagnostic tests with an implicit threshold: a cross sectional study,". BMJ Open. 2 (1): e000746. PMID 22307105.
- Leeflang MM, Bossuyt PM, Irwig L., Diagnostic test accuracy may vary with prevalence: implications for evidence-based diagnosis, J Clin Epidemiol. 2009 Jan;62(1):5-12.
- Goehring C, Perrier A, Morabia A (2004). "Spectrum bias: a quantitative and graphical analysis of the variability of medical diagnostic test performance". Statistics in medicine 23 (1): 125–35. doi:10.1002/sim.1591. PMID 14695644.
- Fletcher RH (1986). "Carcinoembryonic antigen". Ann. Intern. Med. 104 (1): 66–73. PMID 3510056.
- Lachs MS, Nachamkin I, Edelstein PH, Goldman J, Feinstein AR, Schwartz JS (1992). "Spectrum bias in the evaluation of diagnostic tests: lessons from the rapid dipstick test for urinary tract infection". Ann. Intern. Med. 117 (2): 135–40. PMID 1605428.
- Rutjes AWS, Reitsma JB, Vandenbroucke JP, Glas AS, Bossuyt PMM, Case-control and two-gate designs in diagnostic accuracy studies, Clin Chem 2005;51(8):1335-41.
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