What is a limitation of presumptive tests?

Presumptive tests are designed to quickly indicate the presence of a substance, often in forensic and clinical settings. One of the notable limitations of these tests is their tendency to produce false positives. This means that a test might suggest the presence of a specific substance even when it is not actually present. This can occur due to various factors, such as the presence of other similar substances or the sensitivity of the test to non-target compounds.

The reliability of presumptive tests is often assessed through follow-up confirmatory tests, which can provide more definitive identification. This characteristic highlights the importance of using presumptive tests as a first step rather than a definitive diagnosis, as they can lead to potentially misleading conclusions if not further validated with more specific methods.

In contrast, other options are not true limitations of presumptive tests: they can be used on various biological samples, they can indicate the presence of substances even if they are not definitive, and they are not restricted solely to illegal substances.