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• Monitoring urine output with nappy weighing: further considerations
  Saroj Kumar Patnaik
  Published on: 22 March 2016
• Measuring urine output in neonates: the second best option
  Ranjeet W Thergaonkar
  Published on: 22 March 2016

• Published on: 22 March 2016

  Monitoring urine output with nappy weighing: further considerations

  • Saroj Kumar Patnaik, Paediatrician
  • Other Contributors:
    • Suprabha K Patnaik

  Dear Editor

  The study of Dr Oddie et al. interestingly demonstrates no clinically relevant evaporative loss from nappies over a 6 hourly nursing cycle in ELBW babies cared for in some humidity.[1]

  However, in a typical clinical context, the frequency and desirable level of accuracy of urine output measurement needs further contemplation. Oliguria in VLBW babies may be normal in the first 24 hours...

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  Dear Editor

  The study of Dr Oddie et al. interestingly demonstrates no clinically relevant evaporative loss from nappies over a 6 hourly nursing cycle in ELBW babies cared for in some humidity.[1]

  However, in a typical clinical context, the frequency and desirable level of accuracy of urine output measurement needs further contemplation. Oliguria in VLBW babies may be normal in the first 24 hours of life followed by a diuretic phase on day 2 to 3. Irrespective of the gestational age, a urine output of 1-3 ml/kg/hr is desired with <_0.5- _1="_1" ml="ml" kg="kg" hr="hr" suggestive="suggestive" of="of" oliguria="oliguria" and="and"/> 5 ml/kg/hr denoting polyuria.[2] While in a stable baby, 8-12 hourly monitoring suffices, a 6 hourly assessment becomes relevant in the diuretic phase or if the baby is critically ill which is addressed by the present study.

  Extrapolation of the results of this study lead to some interesting conclusions. The 6 hourly measured outputs assuming a loss of 46% (2.3 g out of 5 g urine in a dry environment at 39 degree C as noted in Oddie et al study) as against the calculated urine outputs is given below in the sequence.

  Wt	UOa(6)	UOb(6)	UOc6	D5	D1	D0.5
  500	0.135(0.81)	0.27(1.62)	9.72	5.28	1.38	0.69
  600	0.162(0.97)	0.32(1.94)	11.66	6.34	1.66	0.83
  650	0.176(1.05)	0.35(2.12)	12.64	6.86	1.79	0.90
  700	0.189(1.13)	0.39(2.27)	13.61	7.39	1.93	0.97
  750	0.203(1.22)	0.42(2.43)	14.58	7.92	2.07	1.04
  850	0.230(1.38)	0.46(2.75)	16.52	8.98	2.35	1.17
  1000	0.270(1.62)	0.54(3.24)	19.44	10.56	2.76	1.38
  1500	0.405(2.43)	0.81(4.86)	29.16	15.84	4.14	2.07
  1800	0.486(2.92)	0.97(5.83)	34.99	19.01	4.97	2.48
  2000	0.540(3.24)	1.08(6.48)	38.88	21.12	5.52	2.76
  2250	0.608(3.65)	1.22(7.29)	43.74	23.76	6.21	3.11
  2500	0.675(4.05)	1.35(8.10)	48.60	26.40	6.90	3.45

  Wt=Weight in grams UO a(6)=Urine Output(in ml) at 0.5 ml/kg per hour* (cumulative over 6 hours) UO b(6)=Urine Output (in ml)at 1 ml/kg per hour (cumulative over 6 hours) ** UO c6= Cumulative 6 hours urine output at 5 ml/kg/hr D=Discrepancy over 6 hrs (ml)(expected with no evaporative loss – actual observed with 46% evaporative loss) At 5 ml/kg/hr (D5); 1 ml/kg/hr (D1); 0.5 ml/kg/hr (D0.5)

  * Weight x 0.5 x (100-46/100) = per hour output; for 6 hours multiplied by 06

  ** Weight x 1.0 x (100-46/100) = per hour output; for 6 hours multiplied by 06

  The evident facts are:
  1. With a 6 hourly urine output monitoring, the usual 5 grams scales can detect a urine output of >0.5 ml/kg/hr and 1 ml/kg/hr only if the baby weighs 1675 grams and 850 grams respectively and there is no evaporative loss. With an assumed loss as tabulated above for a urine output of 0.5 ml/kg/hr a 5 grams scale is useless for LBW babies. A 1 ml/kg/hr urine output can be recorded only if the weight is >=1600 grams with a 5 gram scale. On the other hand polyuria of >=5 ml/kg/hr can be easily detected by a 5 grams scale. Thus, undoubtedly in ELBW babies a 1-gram scale is desirable if accurate urine output monitoring by nappy weighing is desired.

  2. Two hourly weighing of nappies with a 1-gram scale as recommended by Cooke et al would still be unable to detect significant weight changes at 0.5 ml/kg/hr in babies smaller than 850 grams (3). Urine output of 1 ml/kg/hr in a baby > 500 gram can be detected at the earliest only if there is no evaporative loss. With maximal losses as calculated as above, 2 hourly weighing still would not be sufficiently sensitive to detect changes in babies below 2000 grams and 1000 grams respectively at 0.5 and 1 ml/kg/hr urine outputs.

  3. Since these calculations are based on a nonhumidified environment with a 46% loss of the weight due to evaporative losses in a 39 degrees centigrade temperature, in a humidified environment the losses are expected to be lesser irrespective of whether the actual losses are linear or nonlinear. Hence, a few working rules can be derived from the present study of Oddie et al. -
  

  • a) A 6 hourly change in nappy weight of >= 1 grams should virtually exclude a urine output <_0.5 ml="ml" kg="kg" hr="hr" in="in" a="a" baby="baby" weighing="weighing" _="_" _650="_650" grams="grams"/>= 2 grams in a baby > 650-1800 kg, >= 3 grams > 1800 grams and > =4 grams in a term baby.
  • b) With a lower limit of 1 ml/kg/hr a weight change of > 2 grams should be acceptable in a 500 grams baby, >=3 grams in a baby of 900 grams, >=4 grams in 1300, >=5 grams in a 1600 grams baby, > 6 grams in >1900 gram baby, > 7 grams in > 2100 gram baby, >= 8 grams in a term baby. Similarly the limits for a polyuric baby are evident from the table.
  • c) Another way to look at it is the discrepancy of baby’s weight and measured urine output by weighing the nappies. A discrepancy of >=5 grams of the baby’s weight and measured nappy weights during a 6 hour period should alert one to a possibility of polyuria or a setting of increased insensible water losses. Varying cutoffs at various weights with a baseline assumption of 46% loss due to evaporative losses are evident from the table. Discrepancy of >=1 grams and > = 2 grams between the baby’s weight loss and change in nappy weights should alert one to look for either increased insensible water losses or need for humidification at an expected urine output level of 1 ml/kg/hr and 0.5 ml/kg/hr respectively.

  4. The environmental humidity factor can be virtually eliminated by using disposable nappies (irrespective of composition) which are immediately sealed in a plastic cover whenever the baby passes urine followed by a cumulative weighing at the end of 06-12 hours as the case may be. Studies have shown upto 2 weeks of delay in the weighing of individually sealed pads does not significantly affect the clinical measurement of weight at a variety of low (5g) or high (50g) volumes.[4] Interestingly this fact can be adapted even in babies being cared under radiant warmers provided the nappies are changed immediately and sealed in impermeable plastic covers. Gouyon et al. have reported 15.9+-6.1% as the maximal loss at 2 hrs in incubator - for a 6 hr period assuming a linear loss 66% of weight loss should occur.[5] Even then the aforementioned working rules based on Oddie et al. study should hold true.

  6. Usage of subjective estimation of urine output from the area in a nappy patch suffers from lack of standardization inherent in varying size and thickness of nappies, absorbent properties of the nappy, effect of gravity and shapes of patches. Moreover wetting due to fecal water and urine may not be differentiated. Cumulative weighing of nappies kept sealed in a water impermeable container/cover using a 1gram scale undoubtedly remains a better method.

  7. Finally, in a critically ill neonate requiring precise urinary output monitoring, the gold standard remains a urethral catheterization [2] – only then can we have a hourly output monitoring which is not feasible with urinary bags/pad collections, look into the trends and anticipate renal shutdown and act appropriately. In a polyuric baby e.g. with salt losing nephropathies, calculated replacement of the fluid and electrolytes may be mandatory and a precise output measurement may be more relevant than the surrogate measures. The assumption of equivalence of urine and saline in the present and similar other studies has an important caveat – urinary specific gravity can be variable depending on the solute load and hence the assumption of 1 ml fluid weighing 1 gram may not always hold true. Urinary specific gravity as a variable also needs to be taken into account in such studies.

  8. A 6-12 hourly difference in the neonatal weight remains the most noninvasive and precise parameter to be used in this population and therapeutic decisions can be based on this single parameter.[2] Frequent assessments of the capillary filling time coupled with the baby’s weight record are the most reliable bedside clinical parameters available in titration of fluid requirements. Additional serum elctrolytes, urea, creatinine with spot urine electrolyte values and blood gas analysis can be resorted to wherever there are concerns about significant dyselectrolytemia and precise replacement is desired.

  References

  1. S Oddie, R Adappa and J Wyllie. Measurement of urine output by weighing nappies Arch. Dis. Child. Fetal Neonatal Ed. 2004; 89: F180-F181

  2. Lin PW, Simmons CF Jr. Fluid electrolyte measurement. In: Cloherty JP, Eichenwald EC, Stark AR, ed. Manual of Neonatal Care.5th Ed.Philadelphia: Lippincott Williams & Wilkins, 2004 101-113.

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

  4. Flisser AJ, Figueroa J, Bleustein CB, Panagopoulos G, Blaivas JG. Pad test by mail for home evaluation of urinary incontinence. Neurourol Urodyn. 2004;23(2):127-9.

  5. Gouyon JB, Sonveau N, d’Athis P, et al. Accuracy of urine output measurement with regular disposable nappies. Pediatr Nephrol 1994;8:88–90

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  Conflict of Interest:
  None declared.

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• Published on: 22 March 2016

  Measuring urine output in neonates: the second best option

  • Ranjeet W Thergaonkar, Pediatrician

  Dear Editor

  I read with interest the article on measurement of urine output by weighing nappies by S Oddie et al.[1] The important points driven home by the authors is the significance of humidity as a factor in this estimation and the practicality of a weighing machine that weighs in increments of 1g instead of 5g.

  A practical point I would like to make is the fact that in India most nurseries are...

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  Dear Editor

  I read with interest the article on measurement of urine output by weighing nappies by S Oddie et al.[1] The important points driven home by the authors is the significance of humidity as a factor in this estimation and the practicality of a weighing machine that weighs in increments of 1g instead of 5g.

  A practical point I would like to make is the fact that in India most nurseries are short on adequate trained manpower and frequent are the situations where weighing diapers each time the baby passes urine may not be possible. In our nursery, we often use modified diapers that are fashioned out of cotton wool sandwiched between gauze. A technique that I have found useful is to pour 5, 10 and 15 ml of water or saline measured by syringe and measure the diameter of the stain it makes on the diaper. A mental impression of the size of this stain as compared with the urine stains allows urine to be estimated in aliquots of “about 5 ml”, “about 10 ml” etc. While it is submitted that this technique lacks accuracy and its validity needs yet to be verified by a scientific study, we are able to exclude with a fair amount of confidence oliguria (<1ml / kg / hr) and polyuria (> 5 ml/kg/hr). No difficulty or miscalculation has been encountered in prescription of fluids based on this, especially in conjuncture with 12 hourly weighing of the baby. While we still prefer to estimate urine output by weighing diapers, we fall back on this technique whenever the nurseries has an overwhelming load. A second technique is the use of plastic pouches that are adherent to the perineum – these have the disadvantage of causing irritation to the perineum.

  Reference

  1. S Oddie, R Adappa, and J Wyllie Measurement of urine output by weighing nappies Arch. Dis. Child. Fetal Neonatal Ed. 2004; 89: F180-F181

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  Conflict of Interest:
  None declared.

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