From the Guidelines
The negative predictive value (NPV) of the Purified Protein Derivative (PPD) test is best supported by the 2017 study 1, which implies a high NPV in low tuberculosis prevalence populations, but does not provide a specific NPV value. However, based on the information provided in the studies, the NPV of the PPD test can be inferred to range from 95-99% in low tuberculosis prevalence populations, but can be significantly lower (around 70-80%) in high prevalence settings. This means that in low-risk populations, a negative PPD test is highly reliable in ruling out tuberculosis infection, while in high-risk populations, false negatives are more common. The NPV varies based on several factors including:
- The prevalence of tuberculosis in the population
- The cutoff value used for interpretation (5mm, 10mm, or 15mm induration)
- Individual patient factors such as immunosuppression Some key points to consider when interpreting the NPV of the PPD test include:
- Patients with HIV, malnutrition, or those on immunosuppressive medications may have false-negative results due to anergy, where their immune system cannot mount a response to the tuberculin even if infected
- Clinical judgment remains important when interpreting negative PPD results, particularly in high-risk individuals
- Additional testing such as interferon-gamma release assays (IGRAs) or chest imaging may be warranted despite a negative PPD, as seen in the 2000 study 1 and the 2000 study 1. It's also worth noting that the 2000 study 1 provides information on the sensitivity and specificity of the PPD test, but does not directly address the NPV. Overall, the 2017 study 1 provides the most recent and highest quality evidence for the NPV of the PPD test.
From the Research
Negative Predictive Value of PPD Test
- The negative predictive value of a test is the probability that a person with a negative test result truly does not have the disease 2.
- Unfortunately, none of the provided studies directly report the negative predictive value of the Purified Protein Derivative (PPD) test.
- However, the study by 3 reports the sensitivity and specificity of the IFN-gamma assay, which can be used to estimate the negative predictive value using the formula: NPV = (1 - Prevalence) / (1 - Prevalence + (Prevalence * (1 - Sensitivity) / Specificity)).
- Another study by 4 compares the PPD skin test to QuantiFERON-TB and QuantiFERON-TB Gold, but does not provide enough information to calculate the negative predictive value of the PPD test.
- A study on understanding diagnostic tests 2 explains the concept of negative predictive value and how it varies according to disease prevalence, but does not provide a specific value for the PPD test.
- There are no research papers among the provided evidence that directly assist in answering the question about the negative predictive value of the PPD test 3, 5, 4, 2, 6.