Falsely raised whole blood chloride caused by systemic absorption of cerium nitrate cream for burns.

Whole blood, serum or plasma chloride is almost exclusively measured by potentiometry with an ion-selective chloride electrode which utilizes membrane selectivity to chloride ions. Other anions such as bromide, iodide and thiosulphate can interfere but usually are not present in high enough concentration to cause significant cross reactivity. A patient from our burns unit had serial chloride measurements on a Radiometer ABL800 blood gas analyser. The results were higher in contrast to plasma measurements on the Abbott Architect Ci8200, which were within reference intervals and in line with the patient's pathophysiological status. This indicated a likely interference with the blood gas analyser chloride estimation. The chloride results on the ABL800 for 3rd, 4th and 5th day after the burn accident were 170, 137 and 119 mmol/L. Corresponding plasma chloride results on the Ci8200 were all around 105 mmol/L. Nitrate was found to be markedly elevated in these samples, and the results were 6.7, 4.9 and 1.1 mmol/L, respectively (reference limit < 0.08 mmol/L). To further demonstrate nitrate was the causative agent, pooled plasma spiked with 7 mmol/L of sodium nitrate caused a rise in the ABL800 chloride from 105 to 202 mmol/L. Later we confirmed that the patient was topically medicated with cerium nitrate cream (Flammacerium®, Sinclair IS Pharma, UK) for his burns. In summary, the results clearly indicated nitrate was the interferent with the ABL800 chloride estimation and the source was the topical burns cerium nitrate cream.

Direct urine ammonium measurement: time to discard urine anion and osmolar gaps.

BACKGROUND: A failure of urine ammonium to increase during acidosis indicates impaired renal acidification, and the urinary ammonium concentration is therefore a useful investigation in determining the cause of a metabolic acidosis. However, urine ammonium measurements are not widely available in routine diagnostic laboratories. This has led to the use of urine anion or osmolar gaps, which are unsatisfactory as surrogates for urine ammonium measurement. METHODS: We evaluated the adaptation of two widely available automated plasma ammonium assays for measurement of urinary ammonium. RESULTS: Both assays showed good recovery and linearity in urine samples spiked with ammonium chloride, and acceptable precision. Urine ammonium concentrations estimated from urinary anion and osmolar gaps showed poor agreement with measured urine ammonium concentrations. CONCLUSIONS: Direct urine ammonium measurements are easily performed with modern autoanalysers by simple adaptation of routine plasma ammonium assays. The use of urine anion and osmolar gaps should be abandoned where direct measurement is available.

The effects of seasonal variation of 25-hydroxyvitamin D on diagnosis of vitamin D insufficiency.

AIMS: To explore the effects of seasonal variation on the diagnosis of vitamin D sufficiency and to determine whether age, gender, and ethnicity modify these effects. METHODS: 21,987 adults had a measurement of serum 25-hydroxyvitamin D (25OHD) at Labplus, Auckland City Hospital, between January 2002 and September 2003, and sine curves were fitted for 25OHD versus day of year to predict the 25OHD nadir for each individual. RESULTS: 48% (range: 30-63%) of individuals had 25OHD <50 nmol/L in the month of measurement, but 63% were predicted to have 25OHD <50 nmol/L in late winter or early spring based on expected seasonal variation. The 25OHD levels required to ensure 25OHD levels >50 nmol/L throughout the year varied substantially by season (in summer at least 60-75 nmol/L), and tended to be higher in men than women, decrease with age, and vary with ethnicity. Mean 25OHD levels were very low (<40 nmol/L) in people of Indian, Middle Eastern, and African descent. CONCLUSION: Seasonal variation in 25OHD affects the diagnosis of vitamin D sufficiency. Clinicians should consider the month of sampling when interpreting the results of 25OHD measurements. In New Zealand, a summertime 25OHD <60-75 nmol/L is generally required to ensure year-round 25OHD levels <50 nmol/L.