TY - JOUR
AU - Nerenz, Robert D
AB - A random urine specimen from a 61-year-old male was collected at an outside clinic for the evaluation of hyponatremia. Although the urine sodium concentration was unremarkable (<20 mmol/L), attempts to measure urine osmolality resulted in a “will not freeze error”; repeat on a separate freezing point depression osmometer yielded the same message. Osmolality is a measure of the number of particles dissolved in a given mass of water. The main contributors to serum and urine osmolality in humans include sodium, chloride, glucose, and urea. Plasma and urine osmolality are typically measured in tandem to assess the ability of the kidneys to concentrate urine or identify additional osmotically active substances. Although osmolality may also be measured by vapor pressure reduction, freezing point depression is preferred because it has the advantage of detecting any solute, including volatile substances that are not detected by vapor pressure osmometers. According to a 2016 College of American Pathologists survey, only 0.5% of surveyed laboratories used vapor pressure osmometers (1). Freezing point depression osmometers function by supercooling the sample several degrees (−7 °C) below its freezing point, converting the liquid sample into an ice slurry. Mechanical agitation then induces crystallization, and a sudden liberation of heat causes a rise in sample temperature that in turn establishes a new liquid–solid-state equilibrium. It is at this equilibrium that the freezing point is monitored and measured by sensitive thermistor probes. Osmolality is expressed as mOsm/kg water and is calculated by dividing the freezing point of the solution by the 1-molal freezing point depression of pure water (−1.86 °C) and multiplied by 1000. To identify the cause of the unfreezable urine, we acquired all urine specimens collected under the same accession number and measured several analytes (Table 1). We observed that while values for cup 1 and pour-off 1 showed good agreement, values generated for pour-off 3 differed approximately by a factor of 2, likely indicating sample dilution. Furthermore, despite nearly identical values for cup 1 and pour-off 1, the 2 containers differed markedly in their measured osmolality. We hypothesized that a volatile substance may have been responsible for the differences. We were able to confirm this suspicion by measuring enzymatic ethanol, which revealed a very high ethanol concentration in pour-off 3 (20210 mg/dL, 4370 mmol/L). Interestingly, the ethanol concentration in cup 1 was much more modest (169 mg/dL, 37 mmol/L), whereas the ethanol concentration in pour-off 1 was undetectable. Table 1. Analysis of urine specimens. Analyte . Units . Cup 1 . Pour-off 1 . Pour-off 3 . BUNa mg/dL 344 345 160 mmol/L 122.8 123.2 57.1 Calcium mg/dL 8.6 8.8 4.2 mmol/L 2.15 2.20 1.05 Phosphorus mg/dL 12.8 12.8 6.8 mmol/L 4.13 4.13 2.20 Magnesium mmol/L 1.54 1.56 0.80 Sodium mmol/L <20 <20 <20 Potassium mmol/L 25.6 26 13.1 Chloride mmol/L 21 21 20 Creatinine mg/dL 98 100 48 μmol/L 8663 8840 4243 Protein g/dL 0.053 0.053 0.024 g/L 0.533 0.539 0.24 Albumin g/dL 0.0267 0.0272 0.0133 g/L 0.267 0.272 0.133 Osmolality mOsm/kg 305 264 Did not freeze Analyte . Units . Cup 1 . Pour-off 1 . Pour-off 3 . BUNa mg/dL 344 345 160 mmol/L 122.8 123.2 57.1 Calcium mg/dL 8.6 8.8 4.2 mmol/L 2.15 2.20 1.05 Phosphorus mg/dL 12.8 12.8 6.8 mmol/L 4.13 4.13 2.20 Magnesium mmol/L 1.54 1.56 0.80 Sodium mmol/L <20 <20 <20 Potassium mmol/L 25.6 26 13.1 Chloride mmol/L 21 21 20 Creatinine mg/dL 98 100 48 μmol/L 8663 8840 4243 Protein g/dL 0.053 0.053 0.024 g/L 0.533 0.539 0.24 Albumin g/dL 0.0267 0.0272 0.0133 g/L 0.267 0.272 0.133 Osmolality mOsm/kg 305 264 Did not freeze a BUN, blood urea nitrogen. Open in new tab Table 1. Analysis of urine specimens. Analyte . Units . Cup 1 . Pour-off 1 . Pour-off 3 . BUNa mg/dL 344 345 160 mmol/L 122.8 123.2 57.1 Calcium mg/dL 8.6 8.8 4.2 mmol/L 2.15 2.20 1.05 Phosphorus mg/dL 12.8 12.8 6.8 mmol/L 4.13 4.13 2.20 Magnesium mmol/L 1.54 1.56 0.80 Sodium mmol/L <20 <20 <20 Potassium mmol/L 25.6 26 13.1 Chloride mmol/L 21 21 20 Creatinine mg/dL 98 100 48 μmol/L 8663 8840 4243 Protein g/dL 0.053 0.053 0.024 g/L 0.533 0.539 0.24 Albumin g/dL 0.0267 0.0272 0.0133 g/L 0.267 0.272 0.133 Osmolality mOsm/kg 305 264 Did not freeze Analyte . Units . Cup 1 . Pour-off 1 . Pour-off 3 . BUNa mg/dL 344 345 160 mmol/L 122.8 123.2 57.1 Calcium mg/dL 8.6 8.8 4.2 mmol/L 2.15 2.20 1.05 Phosphorus mg/dL 12.8 12.8 6.8 mmol/L 4.13 4.13 2.20 Magnesium mmol/L 1.54 1.56 0.80 Sodium mmol/L <20 <20 <20 Potassium mmol/L 25.6 26 13.1 Chloride mmol/L 21 21 20 Creatinine mg/dL 98 100 48 μmol/L 8663 8840 4243 Protein g/dL 0.053 0.053 0.024 g/L 0.533 0.539 0.24 Albumin g/dL 0.0267 0.0272 0.0133 g/L 0.267 0.272 0.133 Osmolality mOsm/kg 305 264 Did not freeze a BUN, blood urea nitrogen. Open in new tab Further discussion with the offsite collecting facility revealed that cup 1 was collected in a nonadditive urine container, labeled accordingly, and poured off to generate pour-off 1 and pour-off 2. Pour-off 2 (omitted from Table 1) appeared to be an ordinary urine cup, yet it was later realized that it contained 25 mL of 50% ethanol as a fixative used for the assessment of urine cytology. This result was not realized initially by laboratory staff because the patient information label was placed directly over a warning label that read “contains 50% ethanol.” While pouring off cup 1 to make pour-off 2, our assumption is that ethanol from pour-off 2 splashed back into cup 1. This could explain the presence of ethanol in cup 1 and the difference in osmolality between cup 1 (305 mOsm/kg) and pour-off 1 (264 mOsm/kg). On arrival in the laboratory, a final pour-off was made for osmolality assessment (pour-off 3). Fortuitously, pour-off 2 was chosen to make pour-off 3, ultimately generating an instrument error and prompting an investigation into the root cause. Although freezing point depression osmometers have an analytical measuring range up to 4000 mOsm/kg and are designed to detect volatile substances, the ethanol concentration present in pour-off 3 (approximately 20% ethanol) was sufficient to prevent freezing. One may suspect ethanol contamination if repeated freezing errors are generated on a freezing point depression osmometer. In the course of our investigation we also found a report demonstrating that certain bacteria are capable of nucleating crystallization and could potentially interfere with the operation of freezing point depression osmometers (2). In summary, this case highlights the importance of several preanalytical practices and procedures designed to minimize cross-contamination and visual cues to ensure correct selection of biological specimen containers. (a) Instead of pouring-off, it is better practice to transfer urine to ethanol-containing cytology cups by pipetting to eliminate the risk of splash back. (b) The caps of cytology cups should be color-coded or given identifying markings to distinguish them from urine containers containing no additives. Lastly, (c) patient-information labeling should be affixed in a manner that does not obscure information on the collection tube including preservatives, anticoagulants, or other additives. Most “laboratory errors” occur during the preanalytical phase of testing, often before the specimen arrives in the laboratory. Robust specimen collection and processing systems are vital to ensure specimen integrity and high quality laboratory results. " Author Contributions:All authors confirmed they have contributed to the intellectual content of this paper and have met the following 4 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. " Authors' Disclosures or Potential Conflicts of Interest:No authors declared any potential conflicts of interest. References 1. College of American Pathologists, Chemistry/Therapeutic Drug Monitoring Participant Survey 2016 ; 74 . 2. Margaritis A , Bassi AS. Principles and biotechnological applications of bacterial ice nucleation . Crit Rev Biotechnol 1991 ; 11 : 277 – 95 . 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TI - The Urine that Would Not Freeze
JF - The Journal of Applied Laboratory Medicine
DO - 10.1373/jalm.2016.022228
DA - 2017-07-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-urine-that-would-not-freeze-41rRRosB5O
SP - 132
EP - 133
VL - 2
IS - 1
DP - DeepDyve
ER -