How Head Lice Is Diagnosed

Head lice are a common problem among school-age children and are diagnosed by spotting the live lice or their eggs, called nits, on the scalp or hair. You can learn how to recognize the lice and nits so you can screen your child and other family members. A special comb is often helpful in this process. The good news is that no known diseases are carried by common head lice. The bad news is that they can make you itch and can easily spread between children and their family members.

Self-Checks/At-Home Screening

The most common sites for lice are around and behind the ears and at the neckline on the back of the head. They can also appear on the eyelashes or eyebrows. Live head lice grow to be about the size of a grain of rice and are dark-colored. They will run from the light, so it's not that easy to see them.

Nits are the eggs lice lay on the hair shaft and they are firmly attached with a cement-like substance. They may be shaped like a teardrop. They are 0.8 millimeters by 0.3 millimeters in size, which the CDC describes as being the size of a knot in a thread. They are yellow to white, but may sometimes be the same color as the hair.

Nits are like lice—it's the warmth of the scalp that keeps them alive. Finding nits isn't enough to determine if there's a current infestation. You only need to be concerned with nits that are within a quarter inch of the base of the hair shaft. Those that are farther from the scalp are likely to have already hatched or be non-viable, but they can remain attached for months. If nits haven't hatched by the time hair grows more than half an inch, they probably won't hatch at all.

Once an active infestation is treated, self-checks must continue to ensure the full removal of nits.

Labs and Tests

If you are unsure about whether or not you have found lice, set up an appointment with your family physician. A healthcare provider is likely to be familiar with the appearance and can take a look for both lice and nits, as well as lice feces. If unsure, the healthcare provider may place a piece of transparent tape over the suspected nits or lice in an effort to collect a sample that can be looked at under a microscope.

Your practitioner may use a Woods lamp to help search for nits. This lamp emits ultraviolet light and is also commonly used by dermatologists to look for bacterial and fungal infections, porphyria, and vitiligo.

The healthcare provider will set up a dark room, turn on the lamp, and hold it about four to five inches from the scalp. The skin and hair do not normally shine under ultraviolet light, but a variety of skin infections and infestations will glow, including lice nits.

To prepare for the test, it's best not to use any hair products, as some may have ingredients that also glow under the lamp. You should not look directly into the light since it can damage your eyes.

Head Lice Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

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Differential Diagnoses

It is easy to confuse dandruff with nits. Dandruff differs from nits in a few ways:

• Size: Dandruff particles are various sizes. Nits are fairly uniform in size.
• Location: Dandruff flakes can be all over the head. Nits tend to be more concentrated in areas where head lice like to live (the base of the neck, behind the ears).
• Staying power: While you may be brushing flakes of dandruff from your shoulders, nits don't fall off. Sometimes they are so stuck that you literally have to pull them off the hair with your fingernail during removal.

Hair casts, which are the remnants of the root of the hair follicle, may also be confused with nits. However, these move freely up and down and the hair shaft. You may also confuse hairspray, gel, and other hair product residue with nits. Some people may also have a psychological condition that leaves them convinced they have bugs crawling on them while they don't have any signs of lice.