Diagnostic Accuracy

The collection and synthesis of clinically relevant data including signs, symptoms, complaints, psychological status and social circumstances help make a formal diagnosis. The challenge is how to make an accurate diagnosis. The likelihood of making an accurate diagnosis increases when a particular sign (or a set of clinical findings) characteristic/typical of a suspected disease is present. In some instances, the absence of a particular sign (or a set of signs) may suggest the presence of the suspected disease.

For example, the presence of radiating pain during SLR, crossed SLR, slump test indicates the presence of lumbar nerve root compression secondary to disc herniation. On the other hand, the absence of pinprick and light touch sensation over the lateral and dorsal aspect of foot indicate the presence of L4 – S1 nerve root lesion on the ipsilateral side.

Furthermore, the absence of radiating pain during SLR, crossed SLR, slump test does not indicate the absence of lumbar disc herniation. Also, the presence of pinprick and light touch sensation over foot does not indicate the absence of L4 – S1 nerve root lesion.

Some findings are useful to make a diagnosis when present and some signs are useful when they are absent. The overall diagnostic accuracy of a clinician depends on the ability to estimate the probability of the presence of a disease. This requires a judgemental use of both positive and negative findings.

To aid the process of making a clinical diagnosis, a variety of special tests, imaging and biochemical test protocols have been developed over the years. The overall diagnostic accuracy of these tests depends on its ability to discriminate between the diseased and healthy state of an individual.

Understanding the principle of diagnostic accuracy requires an introduction to the following concepts:

1. Pre-test probability or prevalence of a disease

2. Sensitivity and Specificity

3. Likelihood ratios

4. Post-test probability

References:

  1. Šimundić, A.M., 2009. Measures of diagnostic accuracy: basic definitions. EJIFCC, 19(4), p.203.
  2. McGee, S., 2012. Evidence-based physical diagnosis. Elsevier Health Sciences.
  3. Fletcher, R.H., Fletcher, S.W. and Fletcher, G.S., 2012. Clinical epidemiology: the essentials. Lippincott Williams & Wilkins.
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