How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. You can treat this by using a warm compress several times a day.
Bleeding can be a problem for people who have bleeding disorders or take blood-thinning medicines such as aspirin or warfarin (Coumadin). If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
A serum osmolality test measures the amount of chemicals in the liquid part (serum) of the blood.
Results are usually available in about 4 hours.
These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.
High levels may be caused by:
- Too little water in the body (dehydration).
- High levels of salt or sugar in the blood. This may be caused by problems such as poorly controlled diabetes.
- Damage to the kidneys. This can cause a buildup of urea in the blood.
- Poisoning with certain substances. These include ethanol (the alcohol in alcoholic drinks), rubbing alcohol (isopropanol), wood alcohol (methanol), and antifreeze (ethylene glycol).
- A rare disease, such as diabetes insipidus, that causes the kidneys to lose water and produce large amounts of urine.
Low levels may be caused by:
- Too much water in the body.
- A low level of salt in the blood. This can be caused by some medicines, including diuretics and certain blood pressure medicines.
- A condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, or the use of certain medicines.