title: Is the refractometer specific gravity a reliable index for pediatric fluid management?
abstract: Serial measurement of urinary output has been one of the mainstays to gauge adequacy of hydration of the newborn surgical patient. Urine output, however, is dependent not only on circulating blood volume but also on osmolar load and the ability of the kidney to excrete solute and water. Therefore, a second measurement is necessary for more accurate monitoring urine concentration. The purpose of this study is to determine the capacity of the kidneys of the newborn surgical patient to dilute and concentrate, and to excrete solute and water; and the accuracy of the refractometer to determine the tonicity of urine. To determine renal capacity, 24 newborn patients were studied during the first 48 hours following major surgical procedures. Analysis of 823 urine samples demonstrated a range of specific gravity between 1.002 and 1.040, and of osmolality between 67 and 582 mosm/kg. The lowest urine output was 0.1 mL/kg/hr and the highest 14.9 mL/kg/hr (mean 3.54 +/- 1.97 mL/kg/hr). The osmolar load excreted by the kidney ranged from 3.7 to 33.4 mosm/kg/24 hr (15.3 +/- 6 mosm/kg/24 hr). To determine the accuracy of the refractometer, 2,007 urine samples from 900 newborn infants were studied. The correlation coefficient for refractometer derived specific gravity and osmolality ranged from 0.7347 to 0.9061. Refractometer specific gravity measurements of 1.008 or below predict hyposmolality as defined as an osmolality below 270 mosm/kg with an accuracy of 70.6% to 95.4%. Accuracy is significantly less in the TPN and the glucose groups. A specific gravity of 1.012 to 1.020 is accurate in predicting hyperosmolality as defined as a urine osmolality above 290 mosm/kg.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D004231:Diuresis; D005069:Evaluation Studies as Topic; D006801:Humans; D007231:Infant, Newborn; D009994:Osmolar Concentration; D011184:Postoperative Period; D012031:Refractometry; D013046:Specific Gravity; D013514:Surgical Procedures, Operative; D014556:Urine / Q000032:analysis*; D014882:Water-Electrolyte Balance