Inaccuracy in neonatal measurement of urine concentration with a refractometer

Serial Free Bisphenol A and Bisphenol A Glucuronide Concentrations in Neonates

OBJECTIVE

To determine the balance of metabolism of free bisphenol A (BPA) to the inactive conjugate, BPA glucuronide (BPAG), in neonates.

STUDY DESIGN

Free BPA and BPAG concentrations were measured in 78 urine samples collected between December 2012 and August 2013 from a cohort of 44 healthy full term (≥ 37 weeks' gestation) neonates at 2 intervals (3-6 days and 7-27 days of age). A questionnaire was administered at the time of sample collection. Neonates recruited into the study were born in an urban, tertiary care hospital.

RESULTS

Only BPAG was detected in the urine samples; concentrations ranged from <0.1 μg/L to 11.21 μg/L (median: 0.27 μg/L). Free BPA concentrations were below the limit of quantification of 0.1 μg/L. Age, but not sex or type of diet, was significantly associated with urinary BPAG concentration (P = .002).

CONCLUSIONS

Our results illustrate widespread BPA exposure in healthy full-term neonates and efficient conjugation of BPA to its readily excretable and biologically inactive form (BPAG) as early as 3 days of age. Factors other than type of diet may be important contributors to BPA exposure in neonates.

The usefulness and limitations of hand-held refractometers in veterinary laboratory medicine: an historical and technical review

Medical hand-held refractometers have been used in veterinary practice since their development in the 1960s. They have become ubiquitous for the measurement of protein and urine solute concentrations because of their rapidity of analysis, ease of use, and relatively low cost. Refraction of light offers advantages for the determination of solute concentrations because the measurement requires no chemical alteration of the specimen. Numerous authors have reported that the results of protein estimation by refractometry for domestic mammals correlate well with those obtained by the biuret method, although others have reported both higher and lower refractometric results compared with biuret results. Major discrepancies between biuret and refractometric results have been reported for avian samples. Some of the variation in reported results may be due to differences in design by refractometer manufacturers. Another possible source may be variation in the biuret reagent mixture and assay conditions. Refractometers also can be used to calculate serum water concentration. A table that converts index of refraction to serum water concentration can be used to convert electrolyte concentration from mmol/L of serum to mmol/L of serum water, a more accurate indicator of effective electrolyte concentration. Refractometers are especially useful for determining urine specific gravity on veterinary samples because they require relatively small sample volumes. Specific gravity continues to be the most common unit for reporting total solids concentration. Some solutes, such as acetone, may cause false increases in specific gravity by refractometry, as they increase refraction but are less dense than water.

Assessment of urine specific gravity by reagent strip test in newborn infants

Specific gravity was measured with a strip test (N-Labstix SG, Ames Division, Miles, Puteaux, France) in 98 urine specimens obtained from 57 newborn infants; osmolality was measured with an osmometer. The strip test did not accurately predict urine osmolalities; a very weak correlation was found between the specific gravity and the osmolality (r2 = 0.598, P < 0.01). Specific gravity values up to 1.015 always indicated hypotonic urine with osmolality less than 211 mosmol/kg H2O, whereas higher values could be associated with either hypotonic or hypertonic urine. Therefore, the strip test cannot be recommended in the neonatal period because its clinical usefulness is strictly restricted to urine samples with low specific gravities (< or = 1.015) and without confounding variables (urine pH > or = 6.5, glucosuria, proteinuria, haematuria).

Special needs of the newborn infant in fluid therapy

The preterm baby, although less able to tolerate high fluid and solute loads, has a remarkable ability to adapt to a rather broad range of environmental challenges, and thus renal function in the preterm infant should no longer be considered impaired. Lower rather than higher fluid administration is the current trend in neonatal care because it is safer and more physiologic. Although changes in body weight over time provide a good indicator of water balance, determination of plasma and urine electrolytes and tonicity are also essential in the monitoring of fluid therapy in preterm infants.

Relative density of urine: methods and clinical significance

The physical properties and chemical composition of urine are highly variable and are determined in large measure by the quantity and the type of food consumed. The specific gravity is the ratio of the density to that of water, and it is dependent on the number and weight of solute particles and on the temperature of the sample. The weight of solute particles is constituted mainly of urea (73%), chloride (5.4%), sodium (5.1%), potassium (2.4%), phosphate (2.0%), uric acid (1.7%), and sulfate (1.3%). Nevertheless, urine osmolality depends only on the number of solute particles. The renal production of maximally concentrated urine and formation of dilute urine may be reduced to two basic elements: (1) generation and maintenance of a renal medullary solute concentration hypertonic to plasma and (2) a mechanism for osmotic equilibration between the inner medulla and the collecting duct fluid. The interaction of the renal medullary countercurrent system, circulating levels of antidiuretic hormone, and thirst regulates water metabolism. Renin, aldosterone, prostaglandins, and kinins also play a role. Clinical estimation of the concentrating and diluting capacity can be performed by relatively simple provocative tests. However, urinary specific gravity after taking no fluids for 12 h overnight should be 1.025 or more, so that the second urine in the morning is a useful sample for screening purposes. Many preservation procedures affect specific gravity measurements. The concentration of solids (or water) in urine can be measured by weighing, hydrometer, refractometry, surface tension, osmolality, a reagent strip, or oscillations of a capillary tube. These measurements are interrelated, not identical. Urinary density measurement is useful to assess the disorders of water balance and to discriminate between prerenal azotemia and acute tubular necrosis. The water balance regulates the serum sodium concentration, therefore disorders are revealed by hypo- and hypernatremia. The disturbances are due to renal and nonrenal diseases, mainly liver, cardiovascular, intestinal, endocrine, and iatrogenic. Fluid management is an important topic of intensive care medicine. Moreover, the usefulness of specific gravity measurement of urine lies in interpreting other findings of urinalysis, both chemical and microscopical.