From the Guidelines
Urine osmolality should be ordered in acute kidney injury (AKI) to help differentiate between prerenal, intrinsic renal, and postrenal causes of kidney dysfunction, as evidenced by the most recent guidelines on renal failure 1. When evaluating a patient with AKI, it is crucial to identify the underlying cause to guide appropriate treatment. The measurement of urine osmolality is a valuable tool in this process.
- In prerenal AKI, urine osmolality is typically high (>500 mOsm/kg) as the kidneys attempt to conserve water by concentrating urine.
- In contrast, intrinsic renal AKI often shows isosthenuric urine with osmolality around 300-350 mOsm/kg, indicating impaired concentrating ability. This test is particularly valuable when interpreted alongside other urinary indices such as fractional excretion of sodium (FENa), urine sodium, and specific gravity, as outlined in the acr appropriateness criteria® renal failure guidelines 1. The physiological basis for these changes relates to tubular function - in prerenal AKI, tubules remain intact and respond appropriately to decreased renal perfusion by increasing water reabsorption, while in acute tubular necrosis or other intrinsic causes, the concentrating mechanism is impaired. Urine osmolality should be measured early in the diagnostic workup, ideally before significant fluid resuscitation or diuretic administration, as these interventions can alter results, and no special patient preparation is required, making it a convenient and readily available test to guide initial management decisions in AKI 1.
From the Research
Importance of Urine Osmolality in AKI
- Urine osmolality is an important parameter in assessing the functioning of the kidneys, especially in patients with acute kidney injury (AKI) 2, 3.
- The measurement of urine osmolality can help in determining the cause of AKI, whether it is prerenal, intrinsic renal, or postrenal 3.
- Monitoring urine output and osmolality can lead to earlier recognition of AKI and better fluid management 4.
Association between Urine Osmolality and AKI
- Studies have shown that high serum osmolality is independently associated with an increased risk of developing AKI 5.
- Low serum osmolality is also associated with an increased risk of AKI, highlighting the importance of monitoring urine osmolality in critically ill patients 5.
- The normal range of serum osmolality is 285-300 mmol/L, and deviations from this range can indicate an increased risk of AKI 5.
Clinical Implications
- Intensive monitoring of urine output, including osmolality, can improve detection of AKI and reduce 30-day mortality in patients experiencing AKI 4.
- Fluid management should aim for early, rapid restoration of circulatory volume, but should be more limited after the first 24-48 hours to avoid volume overload 6.
- Renal replacement therapy should only be started on the basis of hard criteria, but should not be delayed when criteria are met 6.