What happens during an osmolality test?
During a blood test (serum osmolality or plasma osmolality):
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
During a urine osmolality test:
Your health care provider will need to collect a sample of your urine. You will receive a container to collect the urine and special instructions to make sure that the sample is sterile. These instructions are often called the "clean catch method." The clean catch method includes the following steps:
Wash your hands.
Clean your genital area with a cleansing pad given to you by your provider. Men should wipe the tip of their penis. Women should open their labia and clean from front to back.
Start to urinate into the toilet.
Move the collection container under your urine stream.
Collect at least an ounce or two of urine into the container, which should have markings to indicate the amounts.
Finish urinating into the toilet.
Return the sample container to your health care provider.
During a stool osmolality test:
You will need to provide a stool sample. Your provider will give you specific instructions on how to collect and send in your sample. Your instructions may include the following:
Put on a pair of rubber or latex gloves.
Collect and store the stool in a special container given to you by your health care provider or a lab. You may get a device or applicator to help you collect the sample.
Make sure no urine, toilet water, or toilet paper mixes in with the sample.
Seal and label the container.
Remove the gloves and wash your hands.
Return the container to your health care provider or the lab as soon as possible. If you think you may have trouble delivering your sample in time, talk to your health care provider.
