Likelihood ratio calculator

This calculator computes the likelihood ratio for diagnostic tests, helping you evaluate test accuracy by comparing the probability of a positive test result in patients with the disease to those without the disease.

Input Parameters

The proportion of people in the population who have the disease

The probability that the test correctly identifies those with the disease

The probability that the test incorrectly identifies those without the disease

Calculation Results

Likelihood Ratio (LR+)

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Interpretation:
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Post-test Probability

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The probability of having the disease after a positive test result

Calculation Formula

LR+ = (Sensitivity × (1 - Prevalence)) / (1 - Specificity × Prevalence)

Where:
Sensitivity = True Positive Rate
Specificity = 1 - False Positive Rate
Prevalence = Disease Prevalence

Likelihood ratio calculator Calculator Usage Guide

Learn how to use the Likelihood ratio calculator and understand its clinical implications

How to use the calculator

  1. Enter the disease prevalence (the percentage of people in the population who have the disease)
  2. Enter the true positive rate (sensitivity) of the test (the percentage of people with the disease who test positive)
  3. Enter the false positive rate (1-specificity) of the test (the percentage of people without the disease who test positive)
  4. Click the "Calculate" button to see the results

Understanding the results

The calculator provides two key outputs:

  • Likelihood Ratio (+) (LR+): This value indicates how much more likely a positive test result is in patients with the disease compared to those without it.
  • Post-test Probability: This is the estimated probability of having the disease after obtaining a positive test result, given the pre-test probability.

Interpreting Likelihood Ratios

Likelihood ratios are categorized as follows:

  • LR+ < 0.1: Strongly suggests the disease is absent
  • 0.1 ≤ LR+ ≤ 1: Does not help rule in or rule out the disease
  • 1 < LR+ ≤ 10: Suggests the disease is present
  • 10 < LR+ ≤ 30: Strongly suggests the disease is present
  • LR+ > 30: Very strongly suggests the disease is present

Clinical Application

Likelihood ratios are valuable in clinical decision-making because they combine both the pre-test probability and the test's performance to provide an updated probability of disease. They are particularly useful when:

  • Applying clinical decision rules
  • Updating diagnostic suspicion
  • Assessing the value of additional testing

Note: This calculator provides a simplified model for educational purposes. Clinical interpretation should always be done by qualified healthcare professionals considering the full clinical context.