Interpret a health test using the instructions for that product and, for a laboratory result, the reference range supplied with the report. A rapid-test cutoff and a laboratory reference range answer different questions. Check the sample type, control indicator and reading time, and consider symptoms and medical history. A result alone is not a diagnosis.
What do reference ranges and test cutoffs mean?
A laboratory reference range describes results typical of a defined reference population. It differs from a rapid test’s cutoff, which determines how that test classifies a result. Ranges and cutoffs depend on the analyte, sample and method; use the values and instructions supplied with the specific result.
A result within a laboratory reference range is not a guarantee of health, and a result outside it does not automatically establish disease. A rapid test instead reports a result such as positive, negative or invalid according to its specific cutoff and instructions. For example, a hormone screen alone cannot diagnose menopause.
It is important to remember that the reference value is a statistical concept. A small percentage of healthy people can get a result outside the reference values without it being an actual health problem. Therefore, a single test result gives an indication of the situation, but does not solve it on its own.
What is the difference between a positive and negative test result?
The significance of a positive and negative result depends on the principle of operation of the test and the instructions for use. In some tests based on a competitive immunoassay, the absence of a test line may indicate a preliminary positive result, while in other tests, the appearance of a line means a positive result. The interpretation is always made according to the instructions for the product in question and the approved reading time. Neither result alone is a diagnosis.
In a pregnancy test, a positive result means that the hormone hCG has been detected in the urine. In a drug test, a positive result indicates that the substance in question or its breakdown product may be at or above the cutoff of the test. A negative result, on the other hand, means that the substance was not detected with the sensitivity of the method used.
It is always advisable to read the result in parallel with the instructions for use, as the appearance of a positive and negative mark varies from test to test. In many sandwich immunoassays, a faint T line within the reading window can indicate a positive result when the C line is present. An absent C line makes the result invalid. Follow the specific product’s instructions.
Can a home test result be incorrect?
Yes. The result of a rapid test can be affected by, among other things, sampling, test storage, reading time, test restrictions and possible disturbing substances. A rapid test should not be described as equivalent to a laboratory test. An incorrect result can be either a false positive or a false negative, and can occur because of procedural errors, interference or limitations of the test method.
Factors that can cause an incorrect result include:
- Testing too early for the target substance to reach the test’s detection threshold
- Incorrect sample collection or storage
- Not following the product’s instructions for use
- Using an expired test or one exposed to unsuitable temperatures or moisture
- Interference from a medicine or another substance, as described in the product’s limitations
If a result is unclear or unexpected, follow the instructions for repeating the test and seek healthcare advice where appropriate. Repeating a rapid test is not the same as laboratory confirmation. Check the intended use and limitations when choosing health tests.
When does a health test result require a doctor’s assessment?
A doctor’s assessment is always necessary when the result is positive and related to a health condition that may require treatment or further examinations. Even if the result is negative but the symptoms persist, or when the result is unclear, you should see a professional.
Practical examples of situations where a doctor’s assessment is recommended:
- a positive infection or sexually transmitted infection screening result
- hormone-related symptoms that affect quality of life
- a positive pregnancy test when confirmation or advice is needed
- a result that does not fit persistent symptoms
- repeated tests that give different results
A home test can inform your next steps, but it does not replace assessment by a healthcare professional. Seek advice when results are unclear or symptoms persist.
How are the results read differently in different test types?
Different test types display the results in different ways, and the way you read them depends on how the test works. Rapid tests usually show the result visually as lines, symbols, or color changes, while digital tests or tests sent to the lab can provide numerical values.
Reading the Results of a Rapid Test
With rapid tests, such as pregnancy tests or drug tests, the result is read visually after a specified amount of time has elapsed since the sample was added. The instructions for use specify exactly how many minutes after adding the sample the result is ready to be read and how to interpret the different marks. The number and meaning of the lines vary from test to test. Always check the user manual for the specific product to identify the control line, test line, and indicators of an invalid result.
Interpreting Numerical Values
In hormone tests and genetic test reports, results may be presented as numerical values or verbal summaries. The interpretation of tests sent to a laboratory that provide numerical results depends on the method used, the reference range, and the report’s instructions.
Common to all test types is that reading the user manual before performing the test is essential for the correct interpretation. In an unclear situation, the manufacturer or retailer can explain the instructions and product features; a healthcare professional should assess the medical meaning of a result.
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