Although home tests offer convenience, privacy, and “real-time” results, those results are for your personal information. They do not qualify as official test results that could be recorded in your medical record or be acted on by a physician. Results that are used for medical decision making, and recorded as part of your medical history, must be performed by a laboratory or medical professional. Therefore, your doctor is likely to repeat the test that you’ve already done at home before acting on the result.

Errors can arise with any type of home test because of a number of possible mistakes. These range from using an expired test kit to storing the kit inappropriately to how you perform the test. Mistakes in the testing procedure often involve how you collect the sample, the time of day you collect it, or how precisely you time the test (not waiting long enough or waiting too long before noting the result). Even the impact of medications you may be taking may interfere with the results and may be a source of error to be considered.

With the possibility of these various errors, your doctor will most likely want to confirm the result of a home test to ensure its validity before proceeding with any course of action or treatment. While some follow-up procedures from erroneous home test results can be viewed simply as inconvenient, others may be costly or have serious implications. That’s why your doctor will often choose to repeat even a seemingly straight-forward home pregnancy test.

“Many times, results from pregnancy tests may be invalid,” Nichols says. “This is usually because people make mistakes in performing the test.” In a study of pregnancy home use tests sold in France, for example, though nearly all of the negative specimens were interpreted correctly, 53% of urine specimens with pregnancy hormone concentrations at the minimal detection limit were considered negative. And 39% of specimens with twice the detection limit were considered negative. Investigators said that the main problem seemed to be the consumer’s difficulty in interpreting instructions on the package insert and in following those instructions explicitly.

Regarding home testing for blood-thinning medications, Nichols says that clinicians get nervous about tests that monitor medications like coumadin because of the consequences of acting on a wrong result. If the number is erroneously low, inadequate treatment could allow blood to clot. If the number is erroneously high, treatment could cause bruising or internal bleeding.

The importance of repeated (and continuous) monitoring was underscored in a report from a 1999 workshop on the standardization of blood coagulation devices. Alan Jacobson, MD, a panelist at the workshop, said that testing frequency, as much or more than testing accuracy, was responsible for most adverse treatment effects.

“The average Medicare patient on coumadin gets tested only four times per year,” he said. “Cheap testing, but high price in adverse outcomes.”

In contrast, at Jacobson’s institution, patients perform a test that is then verified by additional, separate tests performed by a clinician and a nurse to ensure that the patient can competently perform the test.