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If you see a lump on your guinea pig’s body, schedule an appointment with your vet. Your vet will perform different tests to determine the cause of the lump. Do not delay in taking your guinea pig to your vet—if the lump is an abscess, it could make your guinea pig very sick by overwhelming your guinea pig’s natural defenses against bad bacteria. During your appointment, give your vet as much information as you can about your guinea pig. For example, talk about your guinea pig’s overall health and when you first noticed the lump. In addition, provide information about your guinea pig’s diet, living environment, and if it has cagemates. Your vet will use the information you provide to come up with a diagnosis. Therefore, the more information you provide, the better. A physical examination will give your vet important clues about what caused the lump to form. Your vet will weigh your guinea pig, look it over from head to toe, and listen to its heart and lungs. Your vet will pay special attention to the area of the lump. Your vet may want to run some extra tests to confirm the diagnosis of the lump. One type of test is called fine needle aspiration (FNA), during which your vet will use a small needle to withdraw a fluid sample from the lump. Your vet will look at this fluid under the microscope to detect different cell types (e.g., fat cells, white blood cells). Another type of test is a biopsy, which would help your vet diagnose a tumor.  Your vet would need to sedate or anesthetize your guinea for FNA or a biopsy. A fluid sample from the lump can be useful for identifying bacteria. The test to identify bacteria is called a ‘culture.’
Take your guinea pig to your vet. Explain your guinea pig’s history. Allow your vet to examine your guinea pig. Allow your vet to perform additional testing.