False positives and false negatives in the context of "Iris recognition"

The Great Imitator (also the Great Masquerader) is a phrase used for medical conditions that feature nonspecific symptoms and may be confused with a number of other diseases. The term connotes especially difficult differential diagnosis, increased potential for misdiagnosis, and the protean nature of some diseases. Most great imitators are systemic in nature or have systemic sequelae, and an aspect of nonspecific symptoms is logically almost always involved. In some cases, an assumption that a particular sign or symptom, or a particular pattern of several thereof, is pathognomonic turns out to be false, as the reality is that it is only nearly so.

As recently as the 1950s, syphilis was widely considered by physicians to be "the great imitator", and in the next few decades after that, several other candidates, mainly tuberculosis but occasionally others, were asserted as being "the second great imitator". But because differential diagnosis is inherently subject to occasional difficulty and to false positives and false negatives, the idea that there are only one or two great imitators was more melodrama than objective description. In recent decades, more than a dozen diseases have been recognized in the medical literature as worthy of being considered great imitators, on the common theme of recurring misdiagnoses/missed diagnoses and protean manifestations. Nonetheless, not every caveat (not every mimic) meets the threshold, because it is inherent to differential diagnosis generally that there are thousands of caveats.

Tuberculin, also known as purified protein derivative, is a combination of proteins that are used in the diagnosis of tuberculosis. This use is referred to as the tuberculin skin test and is recommended only for those at high risk. Reliable administration of the skin test requires large amounts of training, supervision, and practice. Injection is done into the skin. After 48 to 72 hours, if there is more than a five to ten millimeter area of swelling, the test is considered positive.

Common side effects include redness, itchiness (pruritus), and pain at the site of injection. Allergic reactions may occasionally occur. The test may be falsely positive in those who have been previously vaccinated with BCG or have been infected by other types of mycobacteria. The test may be falsely negative within ten weeks of infection, in those less than six months old, and in those who have been infected for many years. Use is safe in pregnancy.

Various anti-spam techniques are used to prevent email spam (unsolicited bulk email).

No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to not rejecting all spam email (false negatives) – and the associated costs in time, effort, and cost of wrongfully obstructing good mail. This leads to combinations of the many techniques in order to achieve the best protection against spam and the potential harms that may come with it, while keeping the emails that should be seen intact.

Fine-needle aspiration (FNA) is a diagnostic procedure used to investigate lumps or masses. In this technique, a thin (23–25 gauge (0.52 to 0.64 mm outer diameter)), hollow needle is inserted into the mass for sampling of cells that, after being stained, are examined under a microscope (biopsy). The sampling and biopsy considered together are called fine-needle aspiration biopsy (FNAB) or fine-needle aspiration cytology (FNAC) (the latter to emphasize that any aspiration biopsy involves cytopathology, not histopathology). Fine-needle aspiration biopsies are very safe for minor surgical procedures. Often, a major surgical (excisional or open) biopsy can be avoided by performing a needle aspiration biopsy instead, eliminating the need for hospitalization. In 1981, the first fine-needle aspiration biopsy in the United States was done at Maimonides Medical Center. The modern procedure is widely used to diagnose cancer and inflammatory conditions. Fine needle aspiration is generally considered a safe procedure. Complications are infrequent.

Aspiration is safer and far less traumatic than an open biopsy; complications beyond bruising and soreness are rare. However, the few problematic cells can be too few (inconclusive) or missed entirely (a false negative).