Fluid intake and output

The measurement and documentation of a patient's fluid intake and output are essential aspects of disease monitoring. Changes in fluid intake and output are a valuable gauge of an individual's health status and assist nurses in overseeing the improvement or regression of a disease.

The measurement of fluid intake and output may be undertaken using a variety of means, although these must be accurate and consistent. Fluid input and output, often shortened to fluid balance, may be clinically important in order to:

• establish a baseline prior to surgery or other treatment
• monitor the progression and severity of an illness
• monitor the management of a clinical issues, eg hypotension or shock
• monitor the effectiveness of an intervention, eg diuretic therapy
• estimate fluid loss or gain, eg in kidney disease
• monitor a fluid restriction
• monitor the administration of fluids
• monitoring fluid intake in patients whose drinking may give cause for concern, eg those living with

Nurses must be conversant with the methods of and reasons for measuring and monitoring patient’s fluid balance. They should consider the ways in which measurements are gained, their accuracy, frequency and means of recording. Understanding and being able to respond to changes in an individual’s fluid status may be important in identifying disease progression and remission as well as risk in some patients. 

Individual daily fluid balances are often not as important as trends in fluid balance, so nurses need to consider not only the frequency of measurement, but accuracy and consistency over time. It can be challenging to accurately measure the entirety of an individual’s fluid intake and output, although supplementing fluid balance with daily weights (as 1 litre of fluid equals 1 kilogram) can be helpful (Roumelioti et al, 2018) although this needs to be undertaken consistently and accurately.

If the patient whose fluid balance is to be recorded has

The equipment required for measuring fluid intake and output will depend on the individual and their health status. In patients who have a urinary catheter in situ, measuring output is somewhat simpler than in those patients who do not. However, other than in the high dependency setting, fluid monitoring should not be used as an indication for catheterisation (Shaw et al, 2021). Fluid intake and output monitoring equipment may include:

• catheter drainage bag with additional measuring chamber (for high dependency use)
• measuring jug for urine which may also be used for emptying catheter bags when frequent, eg hourly, accurate measures are not required (supplemented by a urinal bottle or bed pan for patients with limited mobility)
• intravenous fluid giving set with burette or administered via a volumetric pump
• volumetric feeding pump, eg for use with nasogastric or percutaneous endoscopic gastrostomy feed
• drinking vessels, eg cups, mugs and beakers, of a

The nurse should discuss with and explain to the the patient (and if appropriate the family) the rationale for commencing a fluid intake and output assessment. The nurse should attend to any privacy and dignity issues associated with the procedure.

Measuring fluid input must include:

• oral intake
• intravenous intake
• any additional fluids, eg intravenous medications (Dobson and Simpson, 2022)

It is good practice to either record everything hourly on the hour, eg in the high dependency setting, or contemporaneously, eg as the patient finishes a drink. Tips for this include:

• asking the patient to write down everything they drink if they are able
• not recording intake until it has been drunk, ie not recording a drink until it is finished, rather than as it is provided
• enlisting the help of family or visitors in recording fluid intake
• asking about fluid intake regularly
• recording any fluids given as they are given,

Failure to be consistent and accurate with fluid measurement and recording will mean that maintaining a fluid balance for a patient is in essence a waste of time. The biggest risks are that the patient will have an intake of fluid which is not recorded either at all or not accurately. This may include the intake of hidden fluids such as that in gravy and custard which mean estimations of the individual’s intake are poor.

Some of the reasons nurses record fluid intake and output inaccurately include:

• poor training and/or lack of understanding
• lack of time
• low staffing levels (Shepherd, 2011)

Poorly undertaken measurements of fluid intake and output are common as nurses or patients:

• forget to measure their intake or output
• measure the intake or output inaccurately
• cannot measure output accurately as continence products are in use
• urine output is contaminated with faeces

One of the issues which may

Nurses should familiarise themselves with the processes in place where they work for measuring fluid intake and output. Where issues are identified with the process, training and the development of a hydration pathway may be the key to addressing poor practice (Pinnington et al, 2016).

Nursing and Midwifery Council: standards of proficiency for registered nurses

Part 1: Procedures for assessing people’s needs for person-centred care

1. Use evidence-based, best practice approaches to take a history, observe, recognise and accurately assess people of all ages

Part 2: Procedures for the planning, provision and management of person-centred nursing care

5. Use evidence-based, best practice approaches for meeting needs for care and support with nutrition and hydration, accurately assessing the person’s capacity for independence and self-care and initiating appropriate interventions

5.4 record fluid intake and output and identify, respond to and manage dehydration or fluid retention

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Dobson C, Simpson T. Clinical measurement. In Delves-Yates C (ed). Essentials of nursing practice. London: Sage; 2022

Mahon A, Jenkins K, Burnapp L. Oxford handbook of renal nursing.  Oxford: Oxford University Press; 2013

Marieb EN, Keller SM. Essentials of human anatomy & physiology. 12^th edn. London: Pearson; 2017

Pinnington S, Ingleby S, Hanumapura P, Waring D. Assessing and documenting fluid balance. Nursing Standard. 2016;31(15):46-54. https://doi.org/10.7748/ns.2016.e10432 

Roumelioti ME, Glew RH, Khitan ZJ et al. Fluid balance concepts in medicine: Principles and practice. World J Nephrol. 2018;7(1):1-28. https://doi.org/10.5527/wjn.v7.i1.1 

Shaw C, Askins L, Brady G et al. Nutrition and fluid balance. In: Lister S, Hofland J, Grafton H and Wilson C (eds). The Royal Marsden manual of clinical nursing procedures. 10^th edn. Chichester: Wiley Blackwell; 2021:339-402

Shepherd A. Measuring and managing fluid balance. Nursing Times.