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
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
- In rare cases, the vein may become swollen after the blood
sample is taken. This problem is called phlebitis. A warm compress can be used
several times a day to treat this.
- Ongoing bleeding can be a problem for people
with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning
medicines can make bleeding more likely. 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 dissolved in the liquid part (serum) of the blood.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Results are usually available in about 4 hours.
High serum osmolality levels may be
- Too little water in the body (dehydration).
- High levels of salt or sugar
in the blood, such as poorly controlled
diabetes and diabetic coma.
- Damage to the kidneys, which can cause a buildup of
urea in the blood.
- Poisoning with certain
substances, such as ethanol (the alcohol in alcoholic beverages), rubbing
alcohol (isopropanol), wood alcohol (methanol), or antifreeze (ethylene
- A rare disease, such as diabetes insipidus, that causes
the kidneys to lose water and produce large amounts of urine.
Low serum osmolality levels may be
- Too much water in the body
- A low level of salt in the blood, which can be caused by some medicines, including diuretics and some medicines used to treat high blood pressure.
- 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.