Definition/Introduction

Urinary or urine output (UO) is an important clinical indicator of renal physiology and function. Evaluating UO can provide insights into a patient's hydration status and guide subsequent management. A normal UO is approximately 0.5 to 1.0 mL/kg/h. Therefore, an average, healthy male with a body weight of 70 kg should produce around 35 to 70 mL of urine per hour.

In certain disease states affecting the kidney, a disease process called acute kidney injury (AKI) may arise, which can decrease the UO. The Kidney Disease Improving Global Outcomes (KDIGO) criteria for defining an AKI encompasses UO as one of the markers of severity of AKI. According to these criteria, AKI is classified as stage 1 if the UO is less than 0.5 mL/kg/h for 6 to 12 hours, stage 2 if the UO is less than 0.5 mL/kg/h for 12 hours or more, and stage 3 if the UO is less than 0.3 mL/kg/h for at least 24 hours or if there is anuria for at least 12 hours.[1]

In other disease states affecting the kidney, UO may increase, resulting in a condition called polyuria. Polyuria in adults is defined as a UO exceeding 3 L per 24 hours or 50 mL/kg per 24 hours.[2]

Issues of Concern

Although measuring UO is helpful in clinical settings, several factors must be considered before interpreting the results. UO must be accurately measured to analyze fluctuations in urine production throughout the day. The use of catheters is the most accurate method for measuring UO in patients; however, other methods, such as bottle collection, commodes, and pads, are also utilized. In intensive care settings, UO can be measured at hourly intervals, providing the most dynamic UO assessment.[3] The clinical significance of UO assessment also requires accurate fluid input or output charting to comprehensively evaluate the fluid balance of patients.

Patient-specific factors must be considered when interpreting UO. Patients may have comorbidities, such as diabetes, which can influence UO. Furthermore, patients may also be taking medications, such as diuretics, which can affect the UO. Considering these factors is essential when making clinical decisions based on a patient’s UO.

Clinical Significance

Fall in Urine Output

Evaluation of UO bears significant clinical relevance. Decreased UO is one of the criteria defining AKI and can be one of the earliest signs of the condition.[4] Oliguria—defined as less than 0.5 mL/kg/h—has also been associated with an increased mortality rate.[5][6][5] Thus, prompt detection of a decrease in UO can lead to early intervention and prevent disease progression. However, clinical history, physical examination, and other laboratory biomarkers such as urea and electrolytes are required alongside UO assessment to narrow the differential diagnosis and direct medical treatment. 

Rise in Urine Output

An elevated UO is also clinically significant and can indicate other disease processes. Increased UO is often caused by excess IV fluid administration, often unwittingly, if multiple providers are involved in patient care. Arginine vasopressin is an endocrine hormone produced by the posterior part of the pituitary gland and acts on the kidney's tubules to increase water absorption. Most physiological causes of increased UO revolve around suppressed production or resistance to arginine vasopressin. An increased UO may indicate conditions such as central or nephrogenic diabetes insipidus or primary polydipsia.[7]

Measurement of UO is therefore highly beneficial in clinical settings, as a fall or rise in UO may indicate specific disease processes and, thus, influence medical management. 

Nursing, Allied Health, and Interprofessional Team Interventions

Identifying abnormal UO can provide indicators of the underlying disease process, allowing for early intervention. Healthcare professionals involved in patient care should possess the necessary clinical skills to monitor and assess UO, identify abnormalities, and promptly escalate their concerns. Collaboration among all healthcare members of the interdisciplinary team is pivotal to ensure effective communication and improve patient care.[8]

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References

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Section 2: AKI Definition. Kidney Int Suppl (2011). 2012 Mar;2(1):19-36. [PMC free article: PMC4089595] [PubMed: 25018918]

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Ramírez-Guerrero G, Müller-Ortiz H, Pedreros-Rosales C. Polyuria in adults. A diagnostic approach based on pathophysiology. Rev Clin Esp (Barc). 2022 May;222(5):301-308. [PubMed: 34509418]

3.

Macedo E, Malhotra R, Claure-Del Granado R, Fedullo P, Mehta RL. Defining urine output criterion for acute kidney injury in critically ill patients. Nephrol Dial Transplant. 2011 Feb;26(2):509-15. [PMC free article: PMC3108356] [PubMed: 20562094]

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Engoren M, Maile MD, Heung M, Jewell ES, Vahabzadeh C, Haft JW, Kheterpal S. The Association Between Urine Output, Creatinine Elevation, and Death. Ann Thorac Surg. 2017 Apr;103(4):1229-1237. [PubMed: 27717425]

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Macedo E, Malhotra R, Bouchard J, Wynn SK, Mehta RL. Oliguria is an early predictor of higher mortality in critically ill patients. Kidney Int. 2011 Oct;80(7):760-7. [PubMed: 21716258]

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Bianchi NA, Altarelli M, Monard C, Kelevina T, Chaouch A, Schneider AG. Identification of an optimal threshold to define oliguria in critically ill patients: an observational study. Crit Care. 2023 May 30;27(1):207. [PMC free article: PMC10228087] [PubMed: 37254158]

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Nigro N, Grossmann M, Chiang C, Inder WJ. Polyuria-polydipsia syndrome: a diagnostic challenge. Intern Med J. 2018 Mar;48(3):244-253. [PubMed: 28967192]

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Wei H, Corbett RW, Ray J, Wei TL. A culture of caring: the essence of healthcare interprofessional collaboration. J Interprof Care. 2020 May-Jun;34(3):324-331. [PubMed: 31390903]

Disclosure: Hussein Al-Kazwini declares no relevant financial relationships with ineligible companies.

Disclosure: Prathap Kumar Simhadri declares no relevant financial relationships with ineligible companies.