To the Editor In nephrology practice, 24-h urine collection provides valuable information for renal and extrarenal disease diagnosis and management. Although incomplete urine collection is the most common error, we need to pay attention to the methods used to preserve urine samples. Bacterial overgrowth is a concern, as it may affect some laboratory values [1–4]. Therefore, it is desirable for a 24-h urine specimen to be kept at 4 C. However, preservation at room temperature is more practical in both hospitals and homes. Therefore, we examined the effects of temperature and container type on laboratory values in relation to urine sample preservation. Sixteen participants [5 healthy volunteers (mean age 42 ± 6 years, 3 males, 2 females) and 11 inpatients with renal diseases (55 ± 23 years, 9 males, 2 females)] were recruited. Participants voided their second morning urine, and each urine specimen was divided among four containers: (a) a sterilized sealed test tube, (b) another sterilized sealed test tube, (c) a plastic sealed bottle, (d) a plastic measuring cup. Each sample was 20 ml. The samples in container (a) were stored between 1 and 4 C, whereas those in (b), (c), and (d) were stored in a room without an air conditioner. This study was performed between July and September 2011, during which time the outdoor temperature was between 21 and 33 C. Laboratory values were measured on days 0 (sample collection), 1, 3, and 8. Urea, creatinine, uric acid, sodium, phosphate, and total protein values were measured each time. Changes in these laboratory values are shown in Table 1. For the samples in (a), all laboratory values revealed no significant changes during the eight days. For the samples in (b), (c), and (d), there were significant decreases in urea, creatinine, uric acid, and phosphate values on day 8 compared with those on days 0, 1, and 3. The effects of bacteriuria were demonstrated in an 82-year-old male inpatient with nephrosclerosis, whose serum creatinine was 5.8 mg/dl. For this patient, there were decreases in the range of 8–20 % in urea and creatinine values for the samples in (b), (c), and (d) on day 1 compared with those on day 0 [urea: (a) -2.6 %, (b) -13.0 %, (c) -20.0 %, (d) -12.1 %; creatinine: (a) 0 %, (b) -8.3 %, (c) -8.0 %, (d) -9.1 %]. The patient’s urine sediment revealed no white blood cells in the voided urine on day 0. However, urine culture demonstrated Klebsiella pneumoniae, Pseudomonas spp, and Acinetobacter spp in the same sample. After excluding this patient, the percent changes in urea and creatinine on day 1 remained within the range of ±4 %. Although there were several pathogens, including Escherichia coli, Enterococcus faecalis, and Streptococcus species, in the urine culture for some inpatients, there were no significant changes in urea, creatinine, and uric acid on day 1 compared with day 0 for the samples stored at room temperature. In this study, there were a few patients who had a notable decrease in laboratory values on day 1 at room temperature. An inpatient with bacteriuria showed large decreases in urea and creatinine on day 1 in the samples stored at room temperature. It is possible that ureaseproducing pathogens in urine samples decrease urea values [4]. The most common urease-producing pathogens are Proteus, Klebsiella, Pseudomonas, and Staphylococcus species. Although Escherichia coli is a common cause of urinary tract infection, species producing urease are A. Iwata (&) T. Okada T. Nakao Department of Nephrology, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan e-mail: azusa@tokyo-med.ac.jp