Specimen Collection

CC6548 Fluid Deprivation Laboratory Request Form

4.5 mL PST Blood (Preferred)

0.5 mL Paediatric Micro-PST Blood (Preferred)

5 mL Heparin Blood

4 mL Plain Blood

3.5 mL SST Blood

0.5 mL Paediatric Microsample Blood

0.5 mL Paediatric Micro-heparin Blood
Reference Intervals

Units: mmol/kg

Neonate:	270 - 290
Adult:	280 - 300

Uncertainty of Measurement: 4 mmol/Kg

Turnaround Time: Within 8 hours
Diagnostic Use and Interpretation

When osmolality is requested on a sample, this will be measured by an osmometer.

Calculated osmolality

When sodium, urea and glucose results are available, these can be used to calculate an estimated osmolality.

This can be a useful pointer to the presence of ethanol, mannitol or other solutes in the plasma.

The following formula is used in the laboratory computer calculation (ref: Weisberg, Clin Chem (1975) 21/8 1182. ):

(1.87 x Na) + Urea + Glucose + 8.55

Normal range: 275 - 290

An alternative formula is: (Na x 2) + Urea + Glucose.

Osmolar Gap : The osmolar gap is the difference between the measured and calculated serum osmolalities. The normal range for the osmolar gap is 0 - 15. An increased osmolar gap may be due to the presence of mannitol, ethanol, methanol, ethylene glycol, or other substance.

The calculated osmolarity will differ from the measured osmolality in specimens with high lipid (lipemic) or protein (e.g. myeloma) concentrations.
This is due to pseudohyponatraemia, an apparent low measured sodium concentration (see corrected sodium ). Measured osmolality is not affected by an increased lipid or protein.

Contact Information

The chemical pathology team can be reached via email: chemicalpathologist@adhb.govt.nz or via Lablink (09) 307 4949 ext 22000 or 09-3078995

Emails will receive priority attention from the on-call chemical pathologist. Include the patients NHI.

After-hours: contact Lablink (Auckland City Hospital ext. 22000 or 09-3078995) or hospital operator for on duty staff after hours .

• Dr Samarina Musaad (Clinical Lead) : SamarinaM@adhb.govt.nz ext. 22402
• Dr Cam Kyle : CampbellK@adhb.govt.nz ext 22052
• Dr Weldon Chiu : WeldonC@adhb.govt.nz ext. 23427
• Dr Campbell Heron : CHeron@adhb.govt.nz ext. 23427
• Dr Sakunthala Jayasinghe Saku.Jayasinghe@adhb.govt.nz
• Dr Owen Yi WenyiYi@adhb.govt.nz

Last updated at 11:42:03 16/01/2026