A patient with a sodium of 132, a glucose of 250 and a blood urea nitrogen of 45 has a serum osmolality of:
a. Plasma osmolality is calculated by the following formula
b. = 2 [Na+] + [ glucose] / 18 + BUN / 2.8
c. Normal osmolality of body fluid is 275 to 295mO/L H2O.
d. An osmolality above 320 is tolerated poorly.
e. Values greater than 350 may be fatal.
f. The calculated serum osmolality is normally 5 to 8 mO less thancalculated osmolality.
g. So from this formula serum osmolality of the values given in this case is 294.
h. This difference of 5 to 8 mO is called the osmolar discriminant and is due to the presence of anions. E.g. lactate or phosphate.
i. This is usually done in critically ill patients or post operative monitoring of the patient.
j. The advantage of this is that , if it is calculated along with the renal profil, the clinician can differentiate ATN from oligouric percent state.
k. Renal function is usually measured by urine osmolality and plasm osmolality.
l. The urinary to plasma osmolality ratio is calculated and i fit is above 1.7, good concentrating ability is present.
m. The osmolality clearance is calculated by the following formula
n. Cosm = Uosm/ Pos X urine output(1h)
o. The normal value of this is 100-125 ml/h.