After you draw your differential diagnosis and before seeing the patient, you have a pretest probability of the diagnosis that ails the patient. After you obtain a focused history and physical exam, you have gathered information that will help you draw a more accurate post-test probability and narrow your differential diagnosis. The essence of being a doctor does not lie on the tests, therapies, signs, or symptoms, but on how you use them. Every sign and every symptom represents a diagnostic test.

Understanding statistical terms helps us interpret diagnostic test results.

Sensitivity:

• SnOUT: If a test has a high sensitivity and the elicited test is negative, you have essentially ruled OUT the disease.  
• How accurately the test picks up patients WITH disease

Specificity:

• SpIN: If a test has a high specificity and the test is positive, you have essentially ruled IN the disease. 
• How accurately the test picks up patients WITHOUT disease 

As you can see there are limitations when applying sensitivity and specificity on patients since we don't know if the have the disease or not. Predictive values have their limitations as well, these change with prevalence. Therefore, a test used to detect disease in a population with high prevalence would not be adequate in a population with low prevalence. Keep in mind that tests should not be used to replace your clinical judgement.

Prevalence:

• This is your pretest probability, used before you go into the room to see the patient. 

Likelihood Ratio (LR):

• Compares results of patients with disease vs patients without disease
• More accurate than sensitivity and specificity
• Helps you derive the post-test probability
• Takes sensitivity and specificity into account simultaneously 
• When LR >1 it means the probability of disease increases 
• When LR <1 it means the probability of disease decreases 
• When LR = 1, the probability of disease is unchanged 
• Before ordering a test, eliciting a symptom, or finding a sign, ask yourself: How will the absence or presence of this factor change my post-test probability? 
• Use the Fagan Nomogram to determine post-test probability.

Does the post-test probability drawn after your assessment change your threshold?
When the post-test probability falls between test threshold and treatment threshold, further investigation needs to be done. If it lies above the testing threshold it is encouraged to treat, but if it falls below the testing threshold, it is encouraged to pursue a different diagnosis.


Diagnostic and Test Threshold 7