Clinical Chemistry 48: 900-905, 2002;
(Clinical Chemistry. 2002;48:900-905.)
© 2002 American Association for Clinical Chemistry, Inc.
Preservation of Urine for Flow Cytometric and Visual Microscopic Testing
Timo Kouri1a,
Lotta Vuotari2,
Simo Pohjavaara2 and
Pekka Laippala3
1 Oulu University Hospital, Laboratory Administration, PO Box 50, FIN-90029 OYS, Finland.
2 Centre for Laboratory Medicine, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland.
3 Tampere School of Public Health, Tampere University, and Research Unit of Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland.
aAuthor for correspondence. Fax 358-8-315-5541; e-mail timo.kouri{at}ppshp.fi.
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Abstract
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Background: Preservatives that could prevent destruction of cells, casts, and bacteria in urine are of great practical importance because they allow centralization and improvement of accuracy of urine particle counting. We compared two in-house mixtures and one commercial solution, as well as refrigeration, for their ability to preserve urine for both automated analysis (flow cytometry) and visual microscopy.
Methods: Urine specimens were preserved by refrigeration at 4 °C without preservatives (procedure 1); in a lyophilized solution intended to preserve specimens for bacterial culture (Urine C&S tubes; BD Preanalytical Solutions; procedure 2); in 10 mL/L formalin–0.15 mol/L NaCl (procedure 3); in 80 mL/L ethanol–20 g/L polyethylene glycol (procedure 4); and by storage at 20 °C without preservatives (procedure 5). Test strip measurements were used to select specimens positive for leukocyte esterase, hemoglobin, albumin, or nitrite. For 106 consecutive strip-positive specimens, urinalysis was performed by UF-100TM (Sysmex) and by phase-contrast microscopy after Sternheimer supravital staining. Automated analysis was performed at arrival in the morning, on the same day in the afternoon, and after 1 and 3 days. Visual microscopy was performed at arrival and 3 days later.
Results: Urine bacterial counts were well preserved with procedures 1–3, with a false-positive rate of 0.0–3.4% at day 3 vs 28% without preservation (procedure 5). Erythrocytes were poorly preserved for 3 days (
coefficients, 0.24–0.61); after 1 day, fair preservation was seen with procedure 2 ( = 0.78), compared with less favorable preservation with procedure 1 ( = 0.61) or procedure 5 ( = 0.66). Leukocytes were well preserved by all five procedures in the acidic adult urines investigated. Counts of casts and large epithelial cells were artifactually increased by procedure 3. Procedure 2 performed at least as well as refrigeration for specimens analyzed with visual microscopy.
Conclusions: Urine specimens from adults can be stabilized at room temperature for both automated particle analysis and visual microscopy.
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Introduction
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Preservation of formed elements in urine has gained renewed interest since the development of analytical systems that allow automated analysis of erythrocytes, leukocytes, and bacteria in urine (1)(2). If not preserved, urine particles should be examined within 1 h after voiding at ambient temperature or within 4 h if refrigerated to avoid lysis of material (3)(4). Urine leukocyte counts in specimens from children may be questionable after 2–4 h even with refrigeration (5). Traditionally, ethanol (500 mL/L) is used to preserve uroepithelial cells for cytology. Polyethylene glycol (20 g/L) has been added to the ethanol fixative (Saccomanno’s fixative) to improve preservation results (6). Alternative fixatives have also been reported (7). Preservatives have traditionally been used in many countries for specimens requiring microbiologic investigation, i.e., urine bacterial culture (8). Containers prefilled with boric acid alone (9) or in combination with formic acid or other stabilizing media have been used (10)(11). We investigated ways to preserve urine particles for automated analysis with the UF-100TM and for visual microscopy, using both generic and commercial approaches.
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Materials and Methods
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patient urines and preservation procedures
Consecutive morning urine specimens were collected from adult inpatients as a part of their clinical examination at Tampere University Hospital. Test strip measurements (Combur-10 Test M and Miditron M; Roche Diagnostics) were used to select 106 specimens positive for leukocyte esterase, hemoglobin, albumin, or nitrite to enrich the fraction of pathologic findings. Aliquots of 10 mL were taken for five different storage procedures: refrigeration (4 °C) without preservatives (procedure 1); preservation of 10-mL specimens in Urine C&S tubes with lyophilized borate-formate-sorbitol powder (BD Preanalytical Solutions; currently claimed for storage of specimens for urine bacterial culture) at 20 °C (procedure 2); fixation of 9 mL of urine with 1 mL of 100 mL/L formaldehyde supplemented with 1.5 mol/L NaCl at 20 °C (procedure 3); fixation of 9 mL of urine with 1 mL of 800 mL/L ethanol supplemented with 200 g/L polyethylene glycol at 20 °C (procedure 4); and storing the urine specimen without preservatives at 20 °C (background control; procedure 5).
particle analysis
Automated urinalysis was performed by urine flow cytometry (Sysmex UF-100; SYSMEX-EUROPE GmbH) in the manual mode for sample introduction (to avoid lysis of cells caused by the mixing device) (1)(2)(12). In bacterial counting, the earlier software with two separate channels was used (BACT channel, counting particles with a diameter 
2 µm; and HIGH BACT channel, counting particles 0.6 µm). In the results, only counts from the BACT channel are reported. The following time points were chosen to simulate daily workflow: (a) the morning of arrival in the laboratory (day 0, am); (b) the same day in the afternoon (day 0, pm); (c) the following day (day 1); and (d) the 3rd day after the day of specimen arrival to simulate a weekend delay (day 3). Data were collected online and processed with Microsoft Excel spreadsheets (Microsoft Corporation).
In addition, visual microscopic analysis was performed on 200-µL aliquots of noncentrifuged specimens. After 20 µL of Alcian blue-pyronin B supravital stain was added to the urine (13), 1-µL volumes were counted in disposable chambers (Fast-Read 10; Bio-Sigma) by both brightfield and phase-contrast microscopy (Nikon Eclipse E400; Nikon Europe B.V.). Specimens were counted first on arrival, and final preservation was evaluated after storage for 3 days, using the following ordinal scale: A = no destruction of particles; B = destruction detected but classification possible; C = particles detected but difficult to classify; and D = no particles detected. Successful preservation was defined to include categories A and B at day 3.
statistical analysis
Statistics focused on comparing the stability of particle counts in repeated measurements of each tube within each preservation procedure. The Box-Cox transformation was applied to nonnormally distributed results. The following statistical model was used to analyze the variance of serial measurements: y 
Specimen + procedure + time point + (procedure x time point) + error. The main item of interest was the interaction term (procedure x time point), which, if nonsignificant, indicated that the specimens were preserved within a given procedure over different time points. If the term was significant, a pairwise comparison with the original counts of that procedure was performed using Tukey simultaneous tests.
To determine the degree of clinically significant bidirectional changes (from positive to negative, and simultaneously from negative to positive) during the preservation time, results were also analyzed by grouping them into three categories at each clinical cutoff limit. A gray zone of ±20% at the cutoff count was chosen to visualize counting imprecision, which is high because of generally low particle concentrations in urine. Using Microsoft Excel spreadsheets, we calculated the Cohen coefficient to subtract random agreement from paired observations (14). The coefficient varies between -1 (total disagreement) and +1 (complete agreement, complete stability); a value of 0 indicates no agreement (no stability). A coefficient of 0.8 (80% agreement from all obtainable nonrandom disagreement) may be considered as a good, and 0.6 as minimum for a clinical laboratory test (15). An exact test was used to calculate the statistical significance of each obtained coefficient (null hypothesis is random distribution; = 0).
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Results
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Fourteen of 106 specimens (13%) were rejected in flow cytometric analysis because of a high total count, producing an instrument-error message. In analysis of red blood cells (RBCs),
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an additional three to seven specimens were rejected with different preservation procedures because of difficulties in instrumental classification (Discrimination error RBC). Quantitative counts were obtained for bacteria (BACT channel, counting particles with diameter 2 µm), RBCs, white blood cells (WBCs), casts (CAST channel; gated to detect mainly inclusion-bearing granular or cellular casts because hyaline casts are difficult to discern from slime threads with such an analyzer), large epithelial cells (ECs), and small round cells (SRCs; including both epithelial and inflammatory cells).
preservation of initial particle concentrations
After the initial counting (day 0, am), follow-up counts were measured on the same day in the afternoon (day 0, pm), on the following day (day 1) and 3 days later (day 3). The original median and mean values and the ranges for all particle counts are shown in Table 1
. An increment of BACT counts was seen in tubes stored without preservatives at 20 °C (procedure 5) for 1 and 3 days. Bacterial counts were also increased in procedure 4 (ethanol-polyethylene glycol) at 3 days (Table 1
). RBC counts tended to decrease during storage; a statistically significant reduction in counts was seen in RBCs fixed with formaldehyde solution (procedure 3) at 1 and 3 days (Table 1
). Somewhat surprisingly, the mean RBC counts did not decrease at room temperature (procedure 5). Also unexpectedly, WBCs did not show any differences during the follow-up period. Casts and large ECs values were artifactually higher with procedure 3 after 1 or 3 days (Table 1
). Cast counts were higher in procedure 4 than in the other procedures at all time points, but they remained stable during the observation period. Finally, SRC counts showed no significant changes in the mean particle concentrations during follow-up.
bidirectional changes in particle concentrations
During a preservation period, increases and decreases in particle counts may occur, both of which may be clinically significant. These changes may not alter the mean counts for particles at given time points because alterations in the opposite direction may compensate each other. Lysis of RBCs or WBCs creates false-negative (FN) results. Increases in counts may result from bacterial growth, precipitation of new particulate material, or misclassification caused by in vitro changes in particle size or granularity, leading to false-positive (FP) results. Therefore, cross-tables were created to demonstrate changes in counts for individual specimens. Results were divided into positive and negative subgroups for each particle at chosen clinical cutoff concentrations. A gray zone of ±20% at each cutoff count was applied to allow for the analytical imprecision of counting.
Shown in Fig. 1
is an example of RBC counts obtained using preservation procedure 2 (BD tube). The original counts (Day 0, am) form the horizontal grouping shown in columns, whereas each of the follow-up counts (Day 0, pm; Day 1; and Day 3) form the rows of cross-tables. For RBCs, a cutoff gray zone of 8–12 x 106 RBCs/L was used. The stability of particle counts is shown diagonally, with positive, borderline, and negative specimens (shown in bold font) remaining in their original categories in the follow-up. Stability remained uncertain in gray zone counts (6 + 3 + 3 + 3 = 15 specimens) because of statistical imprecision: the UF-100 flow cytometer counts 8 µL of urine, the total count being distributed according to Poisson distribution. FPs consisted of specimens that in the follow-up showed artifactually increased counts; the left lower corner was included in the FPs only. FNs in the follow-up comprised specimens that lost their counts (the upper right corner included only). On day 3, an increase in FP specimens to 4.8% was seen, because of difficulty in discerning RBCs from other small particles (bacteria, noncellular debris, and precipitate). The Cohen coefficient expresses the fraction of nonrandom stability (agreement) for the diagonal (Fig. 1
).
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Figure 1. Example of classification of RBC counts into 3 x 3 cross-tables after repeated measurements of each tube.
Procedure 2 (Urine C&S tube, BD Vacutainer Systems) was chosen to show in detail how data were compiled in Table 2
for all types of particles and procedures. 95% CI, 95% confidence interval for .
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All coefficients observed for all classified elements with each preservation procedure were graded as summarized in Table 2
. The original detailed data are available in an online data supplement available at the Clinical Chemistry Online website (http://www.clinchem.org/content/vol48/issue6). In bacterial counts (BACT gray zone, 160–240 x 106/L), a deterioration in agreement was seen, as expected, for procedure 5 (no preservatives at 20 °C) after 3 days: agreement was only 56%; the FP rate increased to 28.1%; and the coefficient was reduced to 0.31 (95% confidence interval, 0.15–0.47). This preservation was rated as "Poor". Procedure 1 maintained 91% agreement with the original BACT counts for 3 days with = 0.86 and no FP or FN results. Good agreement in BACT counts was also seen with procedures 2 ( = 0.82) and 3 ( = 0.83), which were rated as "Well" preserved. Procedure 4 yielded in a "Fair" preservation only ( = 0.71).
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Table 2. Preservation of particles counted with UF-100 flow cytometer as evaluated from change of category from positive to negative or vice versa.1
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RBC counts were difficult to preserve properly. The day 0 agreement (between counts on day 0, am, and day 0, pm) for RBC counts was 81–88% in all procedures ( coefficients, 0.68–0.78), inferior to that of all other particles. Both FP and FN rates began increasing on days 1–3, depending on the procedure. The best preservation was achieved with procedure 2 (shown in detail in Fig. 1
). At day 1, the coefficient was 0.78 compared with 0.61 for refrigeration (procedure 1) or 0.66 without preservation (procedure 5; data available at Clinical Chemistry Online website). After 3 days, none of the procedures performed optimally ( range, 0.24–0.61; Table 2
). The possible effect of low conductivity (low osmolality) on the lysis of RBCs was assessed in these urines (our correlation coefficient of conductivity to osmolality was r = 0.87 in another sample of 100 patient urines; unpublished data). With procedure 1, five originally RBC-positive specimens were scored as FN after 1 day of storage at 4 °C. The mean conductivity of these FN specimens was 12.9 mS/cm, compared with a mean conductivity of 13.7 mS/cm for the remaining 83 urines investigated (P = 0.20, Student t-test for two groups assuming unequal variances). Isoosmotic urine has a conductivity of 10 mS/cm.
WBCs were well preserved in the measured adult urine specimens at 4 °C for 3 days (agreement, 91.3%; = 0.86; Table 2
). Similar results were obtained with procedures 2–4 at room temperature ( coefficients, 0.84, 0.80, and 0.80, respectively). The extent of deterioration without preservation (procedure 5) was also fairly low in this study ( = 0.67).
Casts were not a problem in preservation, as evaluated from the stability of counts after preservation (Table 2
). Because of the low counts in the gray zone and small amounts of positive specimens (3–6 of 93 samples, depending on the procedure), coefficients remained low because of random variation. Large ECs were fairly well preserved even without preservatives (procedure 5); no differences were seen between procedures 2 and 1. SRCs again showed low coefficients because of the small number of positive specimens (7–8 of 93 cases were positive for SRC), despite an agreement >90% in all cases (Table 2
).
visual microscopy
For visual microscopy, 10 specimens were not followed because of the original low total count (nothing to be seen). At 3 days after collection, procedure 2 (BD solution) preserved most of the sediment particles better or at least as well as the other procedures when classification A or B was considered acceptable (Table 3
). Procedures 1 and 3 were the next best options. Differences in the percentages did not reach statistical significance, mostly because of the low total number of specimens positive for each particle (8–62 cases). Only renal tubular cells were preserved better with procedure 3 (10 mL/L formalin–0.15 mol/L NaCl solution) than with procedure 2. No increase from the background value of 63% (procedure 5) was seen in preservation of hyaline casts by any procedure.
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Discussion
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Preservation of urine is a prerequisite for regional organization of specimen workflow and improvement in the quality (accuracy) of results. We wanted to examine the possibility of stabilizing urine for automated flow cytometry, which otherwise fulfills the clinical need for basic particle analysis of urine (1)(2)(12). Preliminary results from successful preservation with 5 mL/L formaldehyde (final concentration) for automated analysis were available (16).
Mean bacterial (BACT) counts were mostly reliable up to 3 days with procedures 1–3 (Tables 1
and 2
). Procedure 1 (refrigeration) is the current reference preservation procedure for bacterial culture but is not always applicable in routine applications (8). Formaldehyde preservation (procedure 3) does not allow any culturing. With procedure 2 (BD C&S tube), a 3.4% FP rate was observed on day 3 compared with 28% with no preservation (procedure 5). Because of this slight FP rate with procedure 2 at day 3, specimen preservation should not be extended to 3 days (the company currently claims that the product is useful for bacterial culture for 2 days, which was not examined in this study).
Our difficulty in preserving urine RBCs in general was somewhat surprising (Tables 1
and 2
). RBCs were also less well preserved than granulocytes in visual microscopy (Table 3
), indicating the fragility of these cells in urine. Lysis of RBCs was not related to low osmolality in the urines, as judged from the conductivity of the five specimens that lost their RBCs during storage. Preservation of RBCs succeeded slightly better with procedure 2 than with the other procedures for 1–3 days (Fig. 1
and Table 2
). The reason for the stabilizing effect of the BD additives remains uncertain. Procedure 3 (10 mL/L formaldehyde–0.15 mol/L NaCl solution) reduced RBC counts in our hands and created FN results as early as day 1 (Table 1
). Our results agree with those of Hawkins et al. (16), who showed that RBCs shrink in formaldehyde solutions.
WBC preservation succeeded better than expected in our experiments (Tables 1
and 2
). In visual examination, WBCs were identified in 19% of specimens without preservation (procedure 5), but 79% and 82% of specimens could be classified after storage with procedures 1 and 2 for 3 days, respectively (Table 3
). These results may reflect our selection of morning urine from adult patients. Preservation of WBCs is inferior in pediatric (random) specimens (5).
Most urine casts were stable with procedures 1 and 2, but also in specimens without preservatives (Table 2
). Procedures 3 and 4 created artifactual increases in the automated counts (Table 1
). In visual microscopy, most of the hyaline casts were preserved only partially despite the various preservation procedures (Table 3
). Here, repeated automated analysis may have been more gentle than repeated mixing of vials for sediment microscopy on day 3. Casts are known to disintegrate in urine with alkaline pH (17), whereas our specimens were mostly acidic morning urines.
Large (ECs) and small (SRCs) epithelial cells were fairly well preserved in automated counting (Table 2
); only procedure 3 (formaldehyde) created artifactually high counts on day 3 (Table 1
). Detailed visual microscopy of ECs requires proper fixation if specimens are to be studied after 3 days (Table 3
).
In conclusion, we have shown that urine particles can be preserved for regional transportation at room temperature, usually lasting for a maximum of 1 day. Preservation over the weekend should be considered carefully because of the risk of artifactual bacterial growth. For some particles, such as RBCs, preservatives seem to work better than refrigeration. The best preservation results were obtained with a commercial mixture originally developed for preservation of specimens for bacterial culture (10)(11). Undoubtedly, related preservatives have already been used for particle analysis, particularly in countries where microbiology laboratories also count the cells in urine. It must be noted that pediatric specimens, which have low osmolality and, frequently, high pH, were not analyzed in this study. Additional patient populations should also be examined to complete our knowledge on procedures for urine preservation.
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Acknowledgments
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We are indebted to Dr. Wolfgang Hofgärtner of BD Preanalytical Solutions for constructive comments on the manuscript and to Dr. Vladimir Mats of BD Corporate Statistics for help in the evaluation of the significance of the results. This study was supported financially by BD Preanalytical Solutions.
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Footnotes
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1 Nonstandard abbreviations: RBC, red blood cell; WBC, white blood cell; EC, epithelial cell; SRC, small round (epithelial and inflammatory) cell; FN, false negative; and FP, false positive. 
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References
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Abstract
Introduction
