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Defining Nocturnal Polyuria
By Professor Philip van Kerrebroeck
A simple, logical definition of nocturnal polyuria is needed that is applicable in clinical practice to easily identify the need for treatment.
Although nocturnal polyuria is descriptively defined as “the production of an abnormally large volume of urine during sleep”,1 a precise definition that specifies the “abnormally large volume” has been elusive, and more than 15 definitions of nocturnal polyuria have been described in the literature.2 In a 2002 report, we (the standardization subcommittee of the International Continence Society) suggested, as a practical and clinically relevant proposal, that nocturnal polyuria be defined as a urinary output during sleep of 20% or more of the 24-hour total in the young and of 33% or more in the elderly. These figures apply to persons with normal 24-hour urine output and assume a nocturnal sleeping duration of 8 hours.1,3
In an attempt to reach a true definition of nocturnal polyuria – that is, one that informs the need for treatment to avoid physical harm – van Haarst and co-authors proposed a cut-off value based on their analysis of a reference population.4 That population comprised adult volunteers with no urological history nor complaints who completed questionnaires and frequency-volume charts recording 24-hour periods between the first morning micturitions on successive days. The data obtained were used to calculate the nocturnal polyuria index (i.e. the nocturnal urine output as a percentage of the 24-hour urine output). After excluding data from individuals who had general polyuria (24-hour urine output exceeding 2,800 mL as per the ICS definition and assuming an average body mass of 70 kg) or 2 or more episodes of nocturia, the mean nocturnal polyuria index was 29%, and its 95th percentile was at 48%. Overall, the mean nocturnal polyuria index was 30.8% and the 95th percentile 53%. This 95th percentile of 53% was suggested by van Haarst as the cut-off value for identifying individuals with nocturnal polyuria.
In an editorial comment on this study, Paul Abrams contended that this definition of nocturnal polyuria is too severe and thus limits its usefulness.5 For example, a person with a nocturnal polyuria index of 50% and 24-hour urinary volume of 2,000 mL would probably have 3 or more episodes of nocturia, and therefore much reduced quality of life, yet not be considered to have nocturnal polyuria by the definition of van Haarst. Furthermore, the figure of 53% far exceeds the more intuitive 33% that can be deduced by considering that “normal” nocturnal urinary output, which occurs in general over an 8-hour period, is likely to be no more than one third (i.e. 8/24) of the 24-hour total.
This comment raised a key issue for people with nocturnal polyuria: that quality of life is reduced by the resulting nocturia and the need to wake and get out of bed to void perhaps several times during the night. This aspect of nocturnal polyuria was examined in a longitudinal, population-based trial called the Krimpen study. In that study, nocturnal polyuria was defined in two ways: as a nocturnal urine volume greater than 33% of the 24-hour voided volume (NUV33), and as nocturnal urine production greater than 90 mL per hour (NUP90).6,7 The prevalence of nocturnal polyuria differed substantially depending on the definition used and, in older men, ranged from 15% as defined by NUP90 to 80% as defined by NUV33. NUV33 was common with and without nocturia, and van Doorn et al. suggested that NUP90 is a better discriminator of nocturnal polyuria. In a later study of the determinants of nocturia, these authors acknowledged that nocturnal polyuria by either definition (NUV33 or NUP90) is a significant determinant of nocturia and suggested that the aetiology of nocturnal polyuria is a two-step process that reflects both definitions.
NUV33 and NUP90 both have limitations, however. Normal values of the nocturnal polyuria index increase with age, from 14% in younger men to 34% in those older than 65 years, and the figure of 33% that underlies the NUV33 definition of nocturnal polyuria is not associated with a specific age group. It also assumes a sleep duration of 8 hours. On the other hand, NUP90 was derived from a much larger, epidemiological study, but that study included patients with nocturia that was not bothersome. The concept of bother is important because it is what drives a patient to seek medical support. Defining nocturnal polyuria by NUP90 alone means that an increased nocturnal urinary volume may be considered normal and ignores any bother (and thus reduced quality of life) the patient experiences as a result of the greater number of nocturnal voids associated with an increased nocturnal urine output.
I consider that because 8 hours of sleep represents one third (33%) of the 24-hour period, it is logical that a nocturnal voided volume greater than 33% of the 24-hour volume is obviously abnormal, regardless of sleep duration or age, and is likely to be bothersome. NUV33 therefore provides a simple definition of nocturnal polyuria that is easy to apply in clinical practice, and it allows us to identify patients with bothersome nocturia, based on nocturnal polyuria, who may need treatment. However, further study is needed to confirm this suggestion and produce evidence for this practical approach that, in my experience, is clinically relevant.
1. Van Kerrebroeck P, Abrams P, Chaikin D, et al. The standardisation of terminology in nocturia: report from the Standardisation Sub-committee of the International Continence Society. Neurourol Urodyn. 2002;21(2):179-183.
2. Madersbacher S, Cornu J-N. Nocturnal polyuria: it’s all about definition, and be patient! Eur Urol. 2013;63(3):548-550.
3. Van Kerrebroeck P. Standardization of terminology in nocturia: commentary on the ICS report. BJU Int. 2002;90 Suppl 3:16-17.
5. Abrams P. Editorial comment. J Urol. 2012;188(3):873-874; discussion 874.
6. Van Doorn B, Kok ET, Blanker MH, Westers P, Bosch JLHR. Determinants of nocturia: the Krimpen study. J Urol. 2014;191(4):1034-1039.
7. Van Doorn B, Blanker MH, Kok ET, Westers P, Bosch JLHR. Prevalence, incidence, and resolution of nocturnal polyuria in a longitudinal community-based study in older men: the Krimpen study. Eur Urol. 2013;63(3):542-547.