Question

Visit a local pharmacy and select a self-administered test kit (pregnancy, HIV, glucose, etc). Discuss with your class how companies label inappropriately to market a product (ex: xx% “accurate” as opposed to PPV, NPV). Read the fine-print or published literature to actually determine the sensitivity, specificity, PPV and NPV of this device. Now, that you know these terms, how comfortable are you in using any of these test devices at your home?

Answer 1

Companies label inappropriately to market a product. Let us take an example of pregnancy kits -

Pregnancy tests use antibodies to detect human chorionic gonadotropin (hCG). It is an ideal marker of pregnancy since it rises rapidly and consistently in early pregnancy and can be detected in urine. The most advanced home pregnancy test currently available assesses the level of hCG found in urine and claims to provide women with reliable results within just a few weeks of pregnancy.
Today, over 15 different types of home pregnancy test are available to buy, and all the test kits label inappropriately to market their product.

Many tests claim to be highly accurate and capable of detecting pregnancy before the next monthly period is due, although claims such as 8 days prior to menstruation are unrealistic.

* However, Preliminary data on some home pregnancy tests available are presented which indicate that many results do not match the claims made in the package insert and one should be aware that, although all are labelled as CE, there are currently no standard criteria for testing performance and claims.
Healthcare professionals and women should be aware that some of the claims made for home pregnancy tests are INCONSISTENT  and that common definitions and testing criteria are urgently needed.

* SENSITIVITY measures how often a test correctly generates a positive result for people who have the condition that’s being tested for (also known as the “true positive” rate). A test that’s highly sensitive will flag almost everyone who has the disease and not generate many false-negative results.

* SPECIFICITY measures a test’s ability to correctly generate a negative result for people who don't have the condition that’s being tested for (also known as the “true negative” rate). A high-specificity test will correctly rule out almost everyone who doesn't  have the disease and won’t generate many false-positive results.

# It’s important to recognize that sensitivity and specificity exist in a state of balance. Increased sensitivity – the ability to correctly identify people who have the disease — usually comes at the expense of reduced specificity (meaning more false-positives). Likewise, high specificity — when a test does a good job of ruling out people who don’t have the disease – usually means that the test has lower sensitivity (more false-negatives).

Pregnancy kits detects pregnancy by HCG level detection as hcg level increases in pregnancy but both the blood and urine of non-pregnant subjects contain small amounts of HCG.

HCG from the implanting blastocyst first appears in maternal blood around 6-8 days following fertilization; the levels rise rapidly to reach a peak at 7-10 weeks. With most current pregnancy test kits (sensitivity 25 units per litre) urine may reveal positive results 3-4 days after implantation; by 7 days (the time of the expected period) 98% will be positive. A negative result 1 week after the missed period virtually guarantees that the woman is not pregnant. With the present generation of test kits, false positive results due to interfering materials are likely to happen.

The labels are inappropriate and only used for marketing.
PPV is positive predictive value means the individual will truly have the specific diseases or condition.

where as NPV is negative predictive value means the individual will not have the specific disease or condition.