Your health care provider inserts a needle into the skin lesion, which may contain fluid or pus. Fluid is withdrawn. The fluid may be examined under a microscope. A sample of the fluid may also be sent to a lab. There, it is put in a lab dish or other container that contains culture medium and watched for growth of bacteria, virus or fungi.

If the sore is deep, the provider may inject or spray numbing medicine (anesthetic) into the skin before inserting the needle.

You do not need to prepare for this test.

You may feel a pricking sensation as the needle enters the skin.

In many cases, removing the fluid will lessen pressure within the skin lesion and ease pain.

This test is used to find the cause of a fluid-filled skin lesion. It can be used to diagnose skin infections or cancers.

Abnormal results may be a sign of infection caused by bacteria, fungi, or viruses. Cancer cells may also be seen.

There is a small risk of bleeding, mild pain, or infection.

Blijlevens NMA, van der Velden WJFM. Infections in the immunocompromised host: general principles. In: Bennett JE. Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 305.

Marks JG, Miller JJ, Hollins LC. Dermatologic therapy and procedures. In: Marks JG, Miller JJ, Hollins LC, eds. Lookingbill and Marks' Principles of Dermatology. 7th ed. Philadelphia, PA: Elsevier; 2025:chap 4.

Updated by: Elika Hoss, MD, Assistant Professor of Dermatology, Mayo Clinic, Scottsdale, AZ. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.