Acta Pædiatrica ISSN 0803–5253

REGULAR ARTICLE

A comparative study of sanitary napkins and absorbent nappy padsfor urine output measurement in neonatesSourabh Dutta (sourabhdutta@yahoo.co.in), Shiv Sajan Saini, Anil NarangDivision of Neonatology, Department of Pediatrics, Postgraduate Institute of Medical Education and Research, Chandigarh, India

KeywordsMeasurement, Neonate, Sanitary napkins, Urineoutput

CorrespondenceSourabh Dutta, M.D., Ph.D., Additional Professor,Department of Pediatrics, PGIMER, Chandigarh160012, India.Tel: +91-172-2755313 |Fax: +91-172-2744401 |Email: sourabhdutta@yahoo.co.in

Received9 December 2008; revised 16 February 2009;accepted 23 February 2009.

DOI:10.1111/j.1651-2227.2009.01277.x

AbstractAim: To compare sanitary napkins and absorbent nappy pads (ANP) for urine output (UO)

measurement.

Methods: Phase 1: Freshly passed neonatal urine (5, 10 and 15 mL) was poured onto preweighed

sanitary napkins or ANP, which were juxtaposed to the genital area of manikins placed in

incubators/warmers and weighed at 1/2, 1, 2, 4, 5 and 6 hr. Outcome was percentage weight change

from baseline. Phase 2: Five very low birth weight boys in incubators had UO measurement by test

tubes. A sanitary napkin or ANP was co-applied with the test tube for 4 h each. Urine collected in the

test tube was measured and poured on the device, which was reapplied. Weight and wetness were

checked.

Results: Phase 1: Mean urine loss was 8.35, 13.8, 20.1, 25.2, 33.1, 38.7 and 42.6% at 1/2, 1, 2, 3, 4,

5 and 6 h, respectively (repeated measures ANOVA [RM-ANOVA], p < 0.001). Loss was higher with

ANP than sanitary napkins (32.1% vs. 13.4%, two-way RM-ANOVA, p = 0.001). There was less loss

in incubators versus radiant warmers at 6 h (p = 0.09). Phase 2: There was 12.1 and 26% deficit

with sanitary napkin and ANP, respectively. Wetness was felt in one and four cases, respectively.

Conclusion: Urinary losses are less from sanitary napkins than ANPs.

INTRODUCTIONAccurate urine output (UO) measurement is an importantparameter for assessing hydration status and calculatingfluid requirements in the management of neonates, particu-larly so in the case of sick preterm infants. There are severalmethods of measuring UO, urine collection bags (1) andpreweighed absorbent nappy pads (ANPs) (2) or absorbentdiapers being two of the most commonly used modalities.Urine collection bags that stick to the perineal skin are ex-pensive, not always available in resource-poor settings, andmay sometimes leak or damage the skin. In our neonatalintensive care unit (NICU), preweighed ANPs have beenthe method of choice. However, there are concerns aboutleakage of urine from the sides of the pads and evaporativelosses. Nurses in a busy NICU may not be able to attendto a baby as soon as it passes urine and this may result inunderestimation of UO due to evaporation (3).

There is limited literature regarding the evaporative lossesof urine while using ANPs (3,4). Sanitary napkins are de-signed to absorb and retain body fluids in large amounts,without their surface getting wet. They are cheap, widelyavailable, have a standard weight, require no sterilization,are easy to fix either inside a nappy or fasten by a waist-band and are able to cover the pubic to the perineal region

AbbreviationsUO, urine output; ANP, absorbent nappy pad; NICU, neonatalintensive care unit; VLBW, very low birth weight; ANOVA,analysis of variance.

in neonates of all sizes. Thus, a sanitary napkin can be anattractive option for UO measurement. An extensive litera-ture search did not yield any citations regarding the use ofsanitary napkins for UO measurement in newborn infants.We hypothesized that sanitary napkins have less evapora-tive urinary losses and surface wetness as compared to ANPswhen used in neonates.

PATIENTS AND METHODSThis prospective observational study was conducted in atertiary care level III NICU. Phase 1 was done on a manikinand phase 2 was done on live babies. The products comparedwere sanitary napkins (Stayfree Secura Dry R©, Johnson &Johnson Ltd, Mumbai, India) and ANPs (Molinea R©, ElderPharmaceuticals, Mumbai, India).

Phase 1Experiments were performed on a manikin, with one set be-ing performed in an incubator (Drager 8000 NC, Lubeck,Germany) and the other under a radiant warmer (DragerBabytherm, Lubeck, Germany). In the incubator, the evap-orative losses were assessed at two temperatures (30◦ and34◦C) with a fixed humidity of 40%, and under the open careradiant warmer the evaporative losses were assessed at twodifferent heater outputs (30 and 60%). The dry weight of theurine-collecting device (i.e. either sanitary napkin or ANP)was measured on a nappy-weighing scale with least countof 0.1 g (Indosaw, Ambala, India). Freshly passed neona-tal urine was measured in aliquots (5, 10 and 15 mL) and

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Dutta et al. Sanitary napkins for measuring urine output

poured on to the urine-collecting device. The device wasreweighed and recorded as the base line weight. It was ap-plied to the genital area of the manikin and fastened by awaistband to simulate a live baby. The manikin was ‘nursed’in an incubator or radiant warmer in the above-mentionedambient temperature conditions for 6 h. It was reweighedat 1/2, 1, 2, 4 and 6 h and each time reapplied to the genitalarea. Change in weight from baseline was calculated andcompared between groups.

Phase 2The subjects were five very low birth weight (VLBW) babyboys who were nursed in incubators at air temperatures be-tween 32◦ and 33◦C and humidity of 40%. Informed parentalconsent was obtained. To measure the true amount of urinepassed, test tubes were applied to collect urine. Alongsidethe test tube, these babies wore the sanitary napkin orthe ANP sequentially for 4 h each. The urine-collecting de-vice was applied to the genital area and fastened by a waist-band to one side of the test tube. Whenever urine was passedinto the test tube, the amount was measured, the device wasunfastened, the urine was poured on the device, and thetest tube and the device were both reapplied. Urinary losseswere calculated at 1/2, 1, 2, 3 and 4 h. The study was aban-doned in case of temperature instability. If the baby passedmeconium/stools during the observation period, the surfaceof the device was wiped clean and reapplied if the investiga-tor felt that it could be reworn, or the study was abandonedif it was felt otherwise. The wetness of the surface, the lo-cal condition of the skin and the comfort level of the babywere objectively recorded periodically till 4 h. Comfort wasobjectively judged by three parameters, that is, cry, facialfeatures and activity each scored from 0 to 2 with increasingdiscomfort (no cry, consolable crying, inconsolable crying;no agitation, minor changes, grimacing; no activity, someactivity, agitated). A higher score meant more distress.

Outcome measuresIn both phases, the primary outcome was percentage lossof urine over time. In phase 2 the two groups were alsocompared for the wetness of the surface, condition of theskin in contact with the device and comfort of the baby.

Statistical methodsFor data from phase 1, comparison of the evaporative losseswas done with paired t-test. Two-way repeated measuresANOVA was used to compare the two groups with respectto evaporative losses over time. Multivariate linear regres-sion was used to identify the independent predictors for theevaporative losses. Descriptive statistics were used to evalu-ate the wetness, condition of the perineal skin and comfortscores in phase 2.

ResultsPhase 1The percentage loss of urine over 6 h was compared betweenthe two groups (sanitary napkins and absorbent nappy pads)

Table 1 Comparison of the mean of the percentage loss of urine from sanitarynapkins and absorbent nappy pads

Percentage loss of urine from baseline(mean ± SD)

Time of assessment Sanitary napkins Nappy pads p-value∗

1/2 h 3.6 ± 2 13.1 ± 10 0.0041 h 6.1 ± 2 21.4 ± 15 0.0022 h 11.3 ± 5 28.9 ± 17 0.0023 h 14.9 ± 8 35.4 ± 18 0.0024 h 20.1 ± 8 46.1 ± 23 <0.0016 h 26.9 ± 10 58.3 ± 24 <0.001

∗Paired t-test.

Figure 1 Comparison of the rate of loss of urine over time in the two groups.

(Table 1). At all times, the mean percentage loss of urine wassignificantly less from the sanitary napkins than ANPs.

The comparison of the temporal trend of urinary lossesbetween the two groups is shown graphically (Fig. 1). Thelosses from nappy pads are more and the difference is visibleeven at 1/2 h. Moreover, the difference increases with time.

Repeated measure ANOVA was used to compare the dif-ference of urinary losses over time between the two UOmeasurement methods. A significant difference in urinarylosses was observed between sanitary napkins and ANPs.The estimated marginal mean of the percentage loss of urinewas higher with ANP (32.06%) than with sanitary napkin(13.38%) and this difference was statistically significant F(1,22) = 15.64; p = 0.001. The overall mean percentageurine loss increased over the 6 h observation period, it be-ing 8.35% at 1/2 h, 13.8% at 1 h, 20.1% at 2 h, 25.2% at 3 h,33.1% at 4 h, 38.7% at 5 h and 42.6% at 6 h. The differ-ence in the percentage loss at different times was significantF (7,154) = 92.65; p < 0.001. There was a significant in-teraction between the baseline urine volume and the urinecollection method on the urine loss over time F (7,154) =11.43; p < 0.001. The difference between the urinary lossesfrom sanitary napkins and nappy pads was more when thebaseline urine volume was higher.

Table 2 shows the comparison of the mean percentageurinary losses from baseline between the incubator and open

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Table 2 Comparison of the mean percentage urinary losses from the baseline between the incubator and open care

Time at which Mean urinary losses Mean urinary lossesurinary losses from baseline (%) from baseline (%)measured in incubator in open care radiant warmer p-value

30 min 6.86 ± 8.0 9.83 ± 8.9 0.41 h 11.86 ± 14.9 15.72 ± 10.6 0.472 h 16.53 ± 16.1 23.64 ± 13.2 0.253 h 19.86 ± 17.6 30.47 ±16.1 0.144 h 25.64 ±18.1 40.64 ± 22.4 0.095 h 30.5 ± 19.4 46.80 ± 25.2 0.096 h 34.44 ± 20.8 50.69 ± 24.8 0.09

Table 3 Multiple regression model for independent predictors of urine loss by6 h in phase 1

Regression 95%Predictor variable coefficient confidence interval p-value

Urine collection device −0.67 −45.6, −17.1 <0.001Warming device −0.35 −30.5, −2.0 0.03Initial urine volume −0.06 −2.1, 1.4 p = 0.7

care system. There was a trend towards increased urinarylosses when nursed under an open care system compared toincubator.

Multivariate linear regression was performed to find pre-dictors that were independently associated with the percent-age urine loss by 6 h (Table 3). The multiple regression equa-tion was as follows: urine loss by 6 h (expressed at percentfrom baseline) = 32.43 − 0.67 × urine collection device(0/1) − 0.35 × warming device (0/1) − 0.06 × initial urinevolume. Where sanitary napkin = 0 and ANP = 1 for ‘urinecollection device’; and incubator = 0 and radiant warmer =1 for ‘warming device’. After adjustment, initial urine vol-ume was not an independent predictor of urine loss (95%CI −2.1, 1.4 [p = 0.7]).

Phase 2In phase 2, the cumulative loss of urine over 4 h was 12.1and 26% with sanitary napkins and ANPs, respectively. Wet-ness was felt less frequently with sanitary napkins (n = 1)compared to ANP (n = 4). The mean comfort score was 3.6when sanitary napkins were used and 6.4 when ANPs wereused.

DISCUSSIONThis study has shown that neonatal urinary losses from san-itary napkins are significantly less than from ANP, with thecumulative loss from sanitary napkins being approximately27% by 6 h. The use of ANP and nursing under a radi-ant warmer were two factors independently associated withgreater percentage loss of urine over time. Use of sanitarynapkins on VLBW babies nursed in an incubator resultedin less evaporative urinary losses, less wetness and greatercomfort as compared to the use of ANP.

ANPs in the current study had cellulose fluff as an ab-sorbent material. The claimed fluid absorbing capacity was0.15 mL/cm2. Urine-collecting bags, diapers and sometimestest tubes in case of male babies are other methods of urinemeasurement, which have their own advantages and disad-vantages. The skin tends to get cool due to the wet surface ofANPs, and this might cause temperature instability and dis-comfort. Test tubes are applicable to male babies and thereis a chance of spillage. Preweighed diapers are often used indeveloped countries, but appropriate sizes for preterm ba-bies are not available in developing countries and the costis prohibitively high.

There is limited information available in the existing lit-erature regarding urinary losses with the use of UO mea-surement devices (2–4). Oddie et al. (2) studied the effectof humidity on the evaporative losses in diapers placed inincubator at humidity levels of 85, 60, 40% and no addi-tional humidity. No nappy kept in 85% humidity lost weight.Nappies nursed in no additional humidity lost 46% of theinitial volume at 6 h. Cooke et al. (3) determined evaporativelosses while using absorbent infant diapers with measuredamount of water added to it and placing them in an openposition either under radiant warmer or in an incubator upto 120 min. The temperature of the diaper was maintainedat 32.8–33.9◦C. Weight loss was found to be a function oftime and inversely related to the volume of water added. Themean percentage weight loss was greater for 1 mL (82% at60 min) compared with 2.5 mL (82% by 2 h) and 5 mL (75%by 2 h). This finding was in contrast to our study, where thepercentage weight loss was higher with greater initial urinevolumes. Hermansen and Buches (4) determined the rateof water evaporation from open diapers, closed regular dia-pers and open superabsorbent diapers with known amountsof water, when placed under a radiant heater at 100 and50% heater output, respectively, and in room air. Evapora-tion was greatest with open regular diapers (up to 80% ofinitial urine volume by 2 h), less with open superabsorbentdiapers, and least with closed regular diapers. Evaporationwas greatest with 100% heater output, and least in roomair. In the previous studies evaporative losses were, on anaverage, higher than that noticed in our study. This differ-ence could be due to open position of the diaper, less initialamount of liquid used and the use of water instead of actualneonatal urine in the previous studies (3,4). The open po-sition of the diapers exposes the entire inner surface of thediaper to evaporative losses exaggerating the results. Thatdoes not happen in day-to-day monitoring of neonates. Inthe present study, the ANPs and sanitary napkins were fas-tened to a manikin making the study results more life-like.We could not find any comparison of urinary evaporativelosses under incubator and radiant warmer, in published lit-erature.

Decreased urinary losses with the use of the sanitary nap-kins can be explained by the higher fluid-holding capacityof the sanitary napkins. Sanitary napkins are multilayeredstructures that comprise a body contacting liquid-perviouscover layer, an absorbent system and a barrier layer thatprevents fluid entrapped in the absorbent system to egress

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from the inner surface of the sanitary napkin. The absorbentsystem comprises an absorbent core that entraps the liquidpermanently and a transfer layer that transfers the liquid tothe absorbent core by capillary pressure.

One of the limitations of this study was that we did notevaluate the difference between sanitary napkins and ANPsat various levels of humidity in the incubators.

CONCLUSIONWe would like to recommend sanitary napkins, a novelmethod for measuring urine output for urine measurementin NICUs. They would be particularly useful in situationswhere absorbent diapers for neonates of all sizes are com-mercially unavailable or not readily affordable.

CONFLICT OF INTERESTThe authors have no conflict of interest to declare.

References

1. Ochoa Sangrador C, Brezmes Valdivieso MF. GrupoInvestigador del Proyecto. Sample collection methods for urineculture and analysis. An Pediatr (Barc) 2007; 67: 442–9.

2. Oddie S, Adappa R, Wyllie J. Measurement of urine output byweighing nappies. Arch Dis Child Fetal Neonatal Ed 2004; 89:F180–1.

3. Cooke RJ, Werkman S, Watson D. Urine output measurementin premature infants. Pediatrics 1989; 83: 116–18.

4. Hermansen MC, Buches M. Urine output determination fromsuperabsorbent and regular diapers under radiant heat.Pediatrics 1988; 81: 428–31.

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