<prompt> A patient following head injury was admitted in intensive care ward with signs of raised intracranial pressure. He was put on a ventilator and started on intravenous fluids and diuretics. Twenty-four hours later his urine output was 3.5 litres, serum sodium 156 mEqI and serum osmolarity of 316 mOsmkg. The most likely diagnosis based on these parameters is -High output due to diureticsDiabetes insipidusToo much infusion of normal salineCerebral salt retaining syndrome  <response>: Laboratory abnormalities seen in this patientIncreased serum sodium N-135-145 meqdLIncreased serum osmolarity N-275-293 mosmkgPolyuria Urine output 3Lday sometimes assumed to be 70 kg, thecriteria forpolyuria is 3.5LdayUsually when the diagnosis of polyuria is Established the next step is to assess urine osmolalityUrine osmolarity 300 mosmolL Solute diuresis due to diabetes Mellitus or Diuretics Osmotic diuretics and loop diuretics 300 mosmL Water diuretis Now, it should be further evaluated to determine which type of diabetes insipidus is presentIn the question there is no information regarding the urine osmolality and specific gravityThe other clues given in the question --HO head injury-Raised intracranial tension-Patient is on diuretic Usually is given to reduce pressure -L V. fluid administration intracranial The history and the laboratory abnormalities suggests either diabetes insipidus or diuresis due to osmotic diuretics.The features of Hypernatrenzia, Increased osmolality and increased urine output indicates diuresis due to mannitol.Pts with head injury and increased intracranial pressure requires mannitol adminstration to reduce intracranial pressure.Mannitol is an osmotic diuretic, it is freely filtered at glomerulus and undergoes limited reabsorption.Presence of mannitol in the tubular fluid reduces the corticomedullary gradiant and retains water isoosmatically in the proximal tubular fluid.So, the primary function of mannitol is to increase urinary volume.The loss of water in the urine can cause Hypernatremia and increase in serum osmolarity.Whenever mannitol is administered in cases of head trauma to reduce the intracranial tension, serum osmolarity should always be monitored it should never be allowed to go beyond 320 mosmolkg.Diabetes insipidusHypernatremia and increase in Serum osmolarity can also occur in Diabetes insipidus due to increased water loss.Presence of head injury too, suggests Diabetes insipidusBut, this is not a case of diabetes insipidusTo establish the diagnosis of diabetes insipidus urinary osmolarity and specific gravity is needed- urine osmolarity 300 mosmolkg and- specific gravity 1.02Urine output in cases of diabetes insipidus is generally in the range of 6-7 Lday.Here, the urine output is 3.5 Lday which is just adequate to meet the criteria of polyuriaRemember,The pt is on a diruretic i.e., mannitolMannitol will prevent reabsorption of water in Proximal tubule. Diabetes insipidus will prevent reabsorption water in Collecting duct of So the presence of diabetes insipidus in patients on diuretic will lead to huge loss of water through urine which will certainly be much more than 3.5 LdayNote :The patients is on ventilatorSignificant loss of water also occurs through ventilation by evaporation. This may also contribute to hypernatremia and increase in serum osmolarity.Too much infusion of normal salineToo much infusion of normal saline will never increase sodium and serum osmolarity in physiologically normal patientHypernatremia can be caused by administration of hypertonic saline and NaHCO3. 