Blood Osmolality

Osmolality is the test that is used to measure the concentration of the chemical amounts that are present in the fluid section of the blood or serum. Some of the chemicals that can affect the osmolality of the serum are bicarbonate, sodium chloride, sugar and proteins. This test helps to evaluate the water balance in a person’s body. The doctor may conduct this test if the patient is displaying signs of water loss, hyponatremia or poisoning from a harmful substance like methanol, ethanol or ethylene glycol. It can also be conducted if the patient indicates that a problem exists in producing urine. Osmolality can increase in when dehydration increases and can decrease when overhydration occurs.

While conducting a blood osmolality test a blood sample is taken. In the case of healthy people, when the blood osmolality is high, an antidiuretic hormone is released by the body. The doctor can then calculate osmolality in the serum by proceeding to measure the glucose, blood urea nitrogen and sodium levels present in the blood. This hormone causes the patients kidneys to reabsorb water. This leads to more concentrated urine. The reabsorbed water then leads to dilution of the blood thus allowing the blood osmolality to return to normal. In the case where blood osmolality is low, it suppresses the antidiuretic hormone which leads to a reduction in the quantum of water reabsorbed by the kidney. The patient passes the diluted urine, getting rid of all the excess water and increasing blood osmolality.

Normal results are in the range of 280-303 milliosmoles/kilogram. Abnormal results that are higher may indicate diabetes insipidus, dehydration, hyperglycemia, hypernatremia, ethylene glycol poisoning, methanol poisoning, stroke or trauma to the head, renal tubular necrosis or uremia. Results that are lower than the normal level may indicate overhydration, excess fluid intake, hyponatremia, syndrome of inappropriate levels of antidiuretic hormone secretion and the like. Conducting the test is required to detect the presence of poisons like wood alcohol and rubbing alcohol in the patient’s blood. It helps to check the balance between chemicals and water that is dissolved in the blood. It helps in quick detection of severe dehydration or overhydration cases. It helps in checking the production level of the antidiuretic hormone by the hypothalamus. It also aids in measuring the working levels of the kidneys. In cases where the patient has drunk alcohol before the test or has undergone a blood transfusion in the recent past, then the test results can get affected