Test Diagnostics

Original Editor - Tyler Shultz

Top Contributors - Tyler Shultz, Kai A. Sigel, Rachael Lowe, George Prudden and Claire Knott  

Purpose

The purpose of this page is to provide users of Physiopedia a quick reference to commonly used statistics in physical therapy practice.  These statistics are often used to describe the effectiveness of special tests in identifying specific disorders.  Knowing the diagnostic accuracy of special tests is important obtaining an accurate diagnosis, and in turn maximizing treatment outcomes.

Sensitivity

Sensitivity is defined as the ability of a test to identify patients with a particular disorder.[1]  In other words, it represents the proportion of a population with the target disorder that has a positive result with the diagnostic test[2].  Tests that are highly sensitive are most useful for ruling out a disorder, as people who test negative are more likely not to have the target disorder.  "SnNout" is an acronym that can be used to remember that a highly sensitive test and a negative result is good for ruling out the disorder in question.[3]

For example, the Neers Test has been reported to have a sensitivity rating of 0.93 for detecting subacromial impingement.  So, if the test is negative, the examiner can be confident that the patient does not have impingement.

Specificity

Specificity is the ability of a test to identify patients that do not have the disorder in question.[4]  In other words, specificity is the proportion of the population without the target disorder who test negative for the disorder.[5]  Therefore, tests that are highly specific are useful for ruling in a disorder.  The acronym "SpPin" is commonly used to remember that a test with high specificity and a positive result is good for ruling in a disorder.[6]

For example, the Hawkins-Kennedy test for subacromial impingement has been reported by some to have a specificty of 1.00, or 100%. A positive test result is very likely include those people who have impingement.

Likelihood Ratios

Likelihood ratios are an index measurement that combines the sensitivity and specificty values of a specific test.  Likelihood ratios can be used to gauge the performance of a diagnostic test, as it indicates how much a given diagnostic test will lower or raise the pretest probability of the target disorder.[7]  

  • Positive likelihood ratio (+LR) is the proportion of people who test positive and actually have the disorder.  In other words, +LR indicates the shift in probability that favors the existence of a disorder.[8]   +LR is usually calculated by: +LR = Sensitivity / (1 - Specificity)
  • Negative likelihood ratio (-LR) is the proportion of people who test negative and who do not actually have the disorder.  Or, a test with a -LR indicates the shift in probability that favors the absence of the disorder.[9]  -LR is usually calculated by: -LR = (1 - Sensitivity)/Specificity


Interpretation of Likelihood Ratios [10]
    +LR     -LR                                  Interpretation
 > 10.0  < 0.1 Generate large and often conclusive shifts in probability
 5.0 - 10.0  0.1 - 0.2 Generate moderate shifts in probability
 2.0 - 5.0  0.2 - 0.5 Generate small, but sometimes important shifts in probability
 1.0 -2.0  0.5 - 1.0 Alter probability to a small and rarely important degree


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Resources

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References

  1. Sackett, D.L., Straws, S.E., Richardson, W.S., et al. (2000) Evidence-based medicine: How to practice and teach EBM.(2nd ed.) London: Harcourt Publishers Limited.
  2. Dutton, M. (2008). Orthopaedic: Examination, evaluation, and intervention (2nd ed.). New York: The McGraw-Hill Companies, Inc.
  3. Flynn, T.W., Cleland, J.A., Whitman, J.M. (2008). User's guide to the musculoskeletal examination: Fundamentals for the evidence-based clinician. Buckner, Kentucky: Evidence in Motion
  4. Sackett, D.L., Straws, S.E., Richardson, W.S., et al. (2000) Evidence-based medicine: How to practice and teach EBM.(2nd ed.) London: Harcourt Publishers Limited.
  5. Dutton, M. (2008). Orthopaedic: Examination, evaluation, and intervention (2nd ed.). New York: The McGraw-Hill Companies, Inc.
  6. Flynn, T.W., Cleland, J.A., Whitman, J.M. (2008). User's guide to the musculoskeletal examination: Fundamentals for the evidence-based clinician. Buckner, Kentucky: Evidence in Motion
  7. Dutton, M. (2008). Orthopaedic: Examination, evaluation, and intervention (2nd ed.). New York: The McGraw-Hill Companies, Inc.
  8. Jaeschke, R., Guyatt, J.R., Sackett, D.L. (1994). Users guide to the medical literature. III. How to use an article about a diagnostic test. B. What are the results and will they help me in caring for my patients? JAMA, 27: 703-707.
  9. Cleland, J. (2005). Introduction, orthopedic clinical examination: An evidence-based approach for physical therapists. Carlstadt, NJ: Icon Learning Systems, LLC.
  10. Jaeschke, R., Guyatt, J.R., Sackett, D.L. (1994). Users guide to the medical literature. III. How to use an article about a diagnostic test. B. What are the results and will they help me in caring for my patients? JAMA, 27: 703-707.
  11. Physiotutors. Likelihood ratios explained. Available from: https://www.youtube.com/watch?v=50GFq8IQQk0
  12. Physiotutors. Sensitivity & Specificity explained. Available from: https://www.youtube.com/watch?v=UsOv0DcXk6w
  13. Physiotutors. How to calculate sensitivity and specificity. Available from: https://www.youtube.com/watch?v=9f5XgjWpzi0
  14. Physiotutors. How to calculate Positive (PPV) and Negative Predictive Values (NPV) Available from: https://www.youtube.com/watch?v=305ShTgSs7Y