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A gram stain of a skin lesion is a medical laboratory test that uses a number of specific stains on a skin sample to detect the presence of a bacterial infection and identify its type.
A skin lesion is an abnormal lump or sore that affects a localized area of skin that differs in color or texture from the surrounding area of skin. A lesion may cause irritation or discomfort.
The gram stain method of testing is a very popular method for quick diagnosis and identification of bacterial infection.
The test will tell your doctor whether bacteria are present in the skin sample. This is important, because skin infections can also be caused a number of other factors, including:
Bacterial skin infections can be caused by a number of different bacteria, including:
If bacteria are present, the test will indicate whether those bacteria are "gram positive" or "gram negative." This is important because gram-positive bacteria are much more susceptible to penicillin. Some streptococci are gram-positive. Therefore, the gram stain results will help your doctor know how to treat the infection.
Your doctor may recommend a gram stain of a skin lesion test if you are experiencing symptoms of:
No special preparations are needed for this test. Depending on the type of biopsy, you may be advised to discontinue certain medications (such as aspirin, anti-inflammatory medications, and blood thinners). They might interfere with blood clotting.
Usually the test is administered in your doctor’s office. Your doctor will remove a small sample of tissue from the affected area known as a skin lesion biopsy. Depending upon the method being used, your doctor will remove only the outermost layers of skin or a larger amount of the growth.
A few different methods for performing the procedure are described below. The type of biopsy your doctor performs will depend on the type, size, and location of the sore or lump. You will receive a local anesthetic before your skin biopsy, regardless of the type of procedure.
A shave biopsy is the least invasive method. Only the dermis and a bit of the epidermis will be removed.
Before the procedure, you will be given a local anesthetic. During the procedure, your doctor will use a small scalpel to remove a fragment of skin from the affected area. Afterward, that area of skin will be slightly indented. However, the biopsy does not cause any discomfort and you will not need stitches afterwards.
This type of biopsy is recommended for deeper skin lesions. This type of biopsy goes deeper than a shave biopsy, removing epidermis, dermis, and superficial fat.
You will be given a local anesthetic before the procedure. During the procedure, your doctor will use a sharp, hollow instrument to remove a small, round area of skin about the size of a pencil eraser. Sometimes a larger sample may be taken, in which case the area will be closed with sutures.
If your doctor performs this type of skin biopsy, he or she will inject anesthetic directly into the location to be excised. Your doctor will then remove the complete lesion. Once the lesion has been removed, the area will be closed with sutures and pressure will be applied to stop bleeding. If a large area of tissue has been removed, a skin graft may be used to replace it.
Your doctor will perform an incisional biopsy only if he or she is removing part of a large growth. After injecting the site with anesthetic, he or she will cut a piece from the growth and send it to the laboratory for testing. The area will be stitched up. If necessary, the rest of the growth will be removed after diagnosis.
Once the sample reaches the laboratory, it is treated with a number of stains and then examined under a microscope to determine the cause of infection.
Your doctor may order a mucosal or skin culture along with this test to determine if there is a fungus present in the lesion. A viral culture may be ordered to test for viral infections such as herpes simplex. Other testing will be done to detect the presence of cancer.
If your test result is normal, no bacteria have been detected on the skin sample.
If your test result is abnormal, bacteria are present and the skin is infected. In this case, it is likely that the bacteria are causing the infection. The test will indicate what type of bacteria is causing the infection.
Above all, your doctor is trying to find out if you have bacteria that are associated with a serious bacterial skin condition. These conditions include:
Left untreated, a number of these infections can often spread from the skin through the bloodstream and affect other tissues. Diagnosing and treating the infection early can prevent it from causing major damage to your body.
The test results will also enable your doctor to determine if additional tests are necessary to form a diagnosis and suggest treatment.
A few risks are related to a skin lesion biopsy, including:
You should let your doctor know if you:
If your gram stain test reveals the presence of bacteria in your skin, your doctor will prescribe an antibiotic such as penicillin or cephalosporin to treat the infection. If the condition is mild, a topical antibiotic (an ointment applied to the skin) such as amoxicillin or cephalexin will suffice.
If the infection is severe, your doctor will prescribe a course of oral antibiotics, such as bacitracin or mupirocin. In the most severe cases, your doctor will administer the first dose of antibiotic by injection. The course will usually last for 10 days.
In addition, you can take other steps to treat the infection. In some cases, you can apply hot compresses and warm soaks to relieve pain and help draw out the infection. In other cases, you will need to elevate the affected area as much as possible and wrap it with a cool compress. Your doctor will be able to advise you what to do for your specific condition.
Above all, you will need to keep the area clean and dry. Wash with soap that contains an antibacterial agent and fully dry the skin afterwards.
Written by: Megan McCrea
Medically reviewed : Brenda B. Spriggs, MD, MPH, FACP
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