Urinary osmolality: creatinine ratio of spot urine samples relates to body hydration status

Abstract

The objectives of the study were to establish (i) The consistency /inconsistency of early morning spot urine osmolality: creatinine ratio (0: C) in healthy subjects on repeated measurements (ii) the influence of water loading on 0: C (iii) the influence of organophosphate (OP) poisoning on 0: C.

Healthy volunteers (n=30) adequately hydrated (a), a similar group of subjects (n=21) (b), and 32 OP poisoned patients in the leU (c) were used for the 3 phases, (i, ii, iii) of this study. Early morning spot urine samples collected on 4 consecutive days from group (a), spot urine samples collected before and hourly intervals up to 3 hours after 1.0 Iof water loading (group (b) and catheter urine samples of ventilated OP poisoned patients (group (c) were analyzed for osmolality and creatinine concentration.

Group (a) healthy subjects showed a mean O:C ratio of 66.6 mosmol.kg⁻¹/mmol.⁻¹ (mean osmolality = 534.0 mosmol.kg⁻¹, mean creatinine = 9.89 mmol.⁻¹) and this was consistent on the 4 days. Group (b) healthy subjects showed diminishing values for osmolality and creatinine concentration after water loading. However, the 0: e ratio increased from the basal value and remained elevated at 3 hours, The mean 0: C ratios before and 1, ,2, 3 hours after water loading were 72.72, 96.4, 106.4, 85.93 mosmol.kg/mmol.⁻¹". This increase was significant at 1 and 2 hours after water loading. The 32 OP poisoned patients showed a mean 0: C ratio of 110.8 mosmol.kg'l/mmol.⁻¹ (mean osmolality = 352.30 mosrnol.kg", mean creatinine concentration = 4.11 mmol.⁻¹). Of the OP poisoned group a subgroup of compatible age (20-30 yr) patients (n = 8 ) showed a mean O:C ratio of 100.7 mosmolkg⁻¹/mmol⁻¹ on the first day of poisoning which was significantly different from the early morning spot urine O:C ratio of normal subjects (66.63) (p = 0.01).

Results indicated that in adequately hydrated healthy subjects, the spot urine sample is adequate in estimating body hydration status. This was reinforced by the water-loading test in which 0: C ratio increased with increasing hydration, promptly in 1 hour.