Diagnostic utility and cost‐effectiveness of reflex bacterial culture for the detection of urinary tract infection in dogs with low urine specific gravity

Background: Urinary tract infections (UTIs) may be subclinical or difficult to detect in dilute urine as sediment abnormalities may not be observed. In our laboratory, bacterial culture is automatically performed (reflex culture) on samples with urine specific gravity (USG)≤1.013 to increase the likelihood of detecting infection. The value of routine culture of dilute urine, however, has not been fully assessed. Objective: The purpose of this retrospective study was to evaluate the frequency of positive bacterial cultures and analyze the diagnostic utility and cost‐effectiveness of culture compared with routine sediment examination for detecting UTI in dilute urine specimens from dogs. Methods: Urinalysis and concurrent aerobic bacterial culture results were obtained from the electronic medical record system at the University of California–Davis Veterinary Medical Teaching Hospital for samples with USG≤1.013 analyzed from July 1998 through January 2005. Urine collection method, presence of leukocytes and bacteria, bacterial culture results, and clinical diagnosis were recorded. Cost‐effectiveness of reflex culture, based on low USG as the sole criterion, was evaluated. Results: Of 1264 urine specimens, 106 (8.4%) had positive bacterial cultures. Using culture as the gold standard, sediment evaluation had a diagnostic sensitivity of 58.5% and specificity of 98.3% (diagnostic accuracy 94.9%). An additional cost of $60 per patient was incurred, leading to average annual costs of $11,668 for reflex bacterial cultures of all samples with low USG, regardless of collection method. Within our study population, 10 urine samples needed to be cultured for each true positive result. Conclusions: The sensitivity of urine sediment evaluation is low for UTI in dilute urine samples; however, reflex bacterial culture does not appear to be cost‐effective in dogs with USG≤1.013 in the absence of active urine sediment or high clinical suspicion for UTI.     

Interobserver reliability of canine urine specific gravity assessed by analog or digital refractometers

Refractometry is utilized routinely to evaluate canine urine specific gravity (USG) in veterinary clinical settings. We aimed to determine if the magnitude of interobserver reliability when assessing canine USG via refractometry could impact clinical judgment. USG was determined in 38 dogs by 3 registered veterinary technicians (RVTs) using both an optical analog refractometer and a digital refractometer. Summary statistics were reported, interobserver reliability was assessed via intraclass correlation coefficient (ICC) analysis through a 2-way mixed-effects model, and agreement between RVT pairs was compared through Bland–Altman plots. The median analog refractometer USG measurement was 1.018 (range: 1.004–1.040) and for the digital refractometer was 1.0176 (1.0035–1.0357). The analog refractometer average measure ICC was 0.995 (95% CI: 0.992, 0.997; p < 0.001). The digital refractometer average measure ICC was 0.999 (95% CI: 0.999, 1.000; p < 0.001). Strong agreement between all pairs of RVTs was seen via Bland–Altman plots for both analog and digital refractometers, with 95% CIs spanning no more than 0.002 in either the positive or negative direction for all pairings. The interobserver variability in canine USG measurements by RVTs was trivial and did not impact clinical judgment and decision-making.     

Comparison of Multistix PRO dipsticks with other biochemical assays for determining urine protein (UP), urine creatinine (UC) and UP:UC ratio in dogs and cats

BACKGROUND: Urine protein: urine creatinine (UP:UC) ratio determined from the quantitative measurement of protein and creatinine in a single urine sample is the best feasible assessment of clinically significant proteinuria in dogs and cats. A dipstick that measures urine protein, urine creatinine, and UP:UC ratio has been used in human medicine and could have application for veterinary practice. OBJECTIVE: The objective of this study was to compare the Multistix PRO dipstick (Bayer Corporation, Elkhart, IN, USA) to other biochemical methods for determination of urine protein and creatinine, and UP:UC ratio in canine and feline urine. METHODS: A complete urinalysis, including sulfosalicylic acid (SSA) precipitation, was performed on urine samples submitted to our laboratory between February and April 2003 from 100 dogs and 49 cats. Urine protein and creatinine concentrations were determined by the Multistix PRO dipstick using a Clinitek 50 analyzer (Bayer) and compared with the results of SSA precipitation and quantitative biochemical analysis. The UP:UC ratios from the dipstick results (calculated by the Clinitek 50 and also manually) were compared with those calculated from quantitative values. Pearson product-moment correlation analysis and diagnostic sensitivity and specificity (using quantitative results as the gold standard) were determined. RESULTS: For both canine and feline urine, protein and creatinine concentrations determined by the Multistix PRO correlated closely with quantitative concentrations for protein (dogs r = .78, P = .0001; cats r = .87, P = .0001) and creatinine (dogs r = .78, P = .0001; cats r = .76, P = .0001). The Multistix PRO was more sensitive and less specific than SSA precipitation for diagnosing clinically significant proteinuria. UP:UC ratios obtained by manual calculation of dipstick results correlated best with quantitative UP:UC ratios in dogs, and had higher specificity but lower sensitivity for the diagnosis of proteinuria. In cats, UP:UC ratios determined by the dipstick method did not correlate (r = -.24, P = .0974) with quantitative values. CONCLUSIONS: The Multistix PRO, with manual calculation of UP:UC, may be a good alternative for the diagnosis of clinically significant proteinuria in dogs, but not cats. Dipstick creatinine concentration should be considered as an estimate.     

Comparison of a digital and an optical analogue hand-held refractometer for the measurement of canine urine specific gravity

Urine specific gravity (USG) is used clinically as a measure of urine concentration, and is routinely assessed by refractometry. A comparison between optical analogue and digital refractometers for evaluation of canine urine has not been reported. The aim of this study was to compare a digital and an optical analogue hand-held refractometer for the measurement of canine USG, and to assess correlation with urine osmolality. Prospective study. Free-catch urine samples were collected from 285 hospitalised adult dogs, and paired USG readings were obtained with a digital and an optical analogue refractometer. In 50 dogs, urine osmolality was also measured using a freezing point depression osmometer. There was a small but statistically significant difference between the two refractometers (P<0.001), with the optical analogue refractometer reading higher than the digital refractometer (mean difference 0.0006, sd 0.0012). Paired refractometer measurements varied by <0.002 in 91.5 per cent of cases. The optical analogue and digital refractometer readings showed excellent correlation with osmolality (r=0.980 and r=0.977, respectively, P<0.001 in both cases). Despite statistical significance, the difference between the two refractometers is unlikely to be clinically significant. Both instruments provide an accurate assessment of USG in dogs.     

Evaluation of urine specific gravity as a predictor of hypotension during anaesthesia in healthy dogs premedicated with dexmedetomidine

ObjectiveTo investigate the relationship between urine specific gravity (USG) and the risk of arterial hypotension during general anaesthesia (GA) in healthy dogs premedicated with dexmedetomidine and methadone.Study designProspective clinical cohort study.AnimalsA total of 75 healthy client-owned dogs undergoing GA for elective tibial plateau levelling osteotomy.MethodsAfter placing an intravenous catheter, dogs were premedicated with dexmedetomidine (5 μg kg^–1) and methadone (0.3 mg kg^–1) intravenously. After induction of GA with alfaxalone to effect, the bladder was expressed and USG measured. An arterial catheter was placed, and residual blood was used to measure packed cell volume (PCV) and total protein (TP). GA was maintained with isoflurane vaporised in oxygen and a femoral and sciatic nerve block were performed. Arterial blood pressure < 60 mmHg was defined as hypotension and recorded by the anaesthetist. Treatment for hypotension was performed in a stepwise manner following a flow chart. Frequency of hypotension, treatment and response to treatment were recorded. Logistic regression modelling was used to assess the association between USG, TP and PCV and incidence of perioperative hypotension; p < 0.05.ResultsData from 14 dogs were excluded. Of the 61 dogs, 16 (26%) were hypotensive during GA, 15 dogs needed treatment of which 12 were responsive to a decrease in inhalant vaporiser setting. The logistic regression model was not statistically significant (p = 0.8). There was no significant association between USG (p = 0.6), TP (p = 0.4), PCV (p = 0.8) and arterial hypotension during GA.Conclusions and clinical relevanceIn healthy dogs premedicated with dexmedetomidine and methadone and maintained under GA with isoflurane and a femoral and sciatic nerve block, there was no relationship between the specific gravity of urine collected after premedication and intraoperative arterial hypotension.     

Development of correction formulas for canine and feline urine specific gravity measured using a Japanese refractometer

One of the most important functions of the kidney is to concentrate urine through the reabsorption of water. Urine specific gravity (USG) is used in routine tests of urine concentration and can be estimated using a refractometer. However, as the scale of Japanese refractometer is based on experimental data from healthy Japanese people, and human USG obtained by Japanese refractometers show higher values than that by refractometer produced in Europe or the U.S.A. The purpose of this study was to establish correction formulas for the USG of dogs and cats measured using Japanese refractometers. In this study, we found that Japanese refractometers overestimated USG in both dogs and cats. This study shows that the correlation formulas described in this study are useful for the accurate evaluation of USG.     

Comparison of refractometer and biuret methods for total protein measurement in body cavity fluids

Abstract: Most hand-held medical refractometers have internal scales that limit protein measurement to results ≥ 2.5 g/dL. Tables for conversion of refraction (r) to protein concentration for values as low as 0.1 g/dL were published in the 1960s, but their accuracy for use on body fluids has not been established. The purpose of this study was to assess the reliability of body cavity fluid protein determination by refractometry. We compared the protein concentration of 25 body cavity fluids as determined by 2 Goldberg type hand-held refractometers with results obtained by the biuret method. Published charts converting refraction (r) to protein concentration were used to determine protein concentration in samples with protein <2.5 g/dL. Higher protein values were read directly from the instruments. The range of comparison was limited to ≥ 0.6 g/dL, the lowest concentration of the biuret method's standard curve. Twenty-one peritoneal fluid, 2 pleural fluid and 2 pericardial fluid samples from 16 horses, 5 cattle, 3 dogs, 2 llamas and 1 cat were tested. The results obtained by the two refractometers were closely and linearly related to biuret results (P<.001), with slopes by linear regression analysis close to 1, and correlation coefficients >0.977. Based on this study, the range for quantification of body cavity fluid protein concentration by refractometry can be extended below 2.5 g/dL, allowing for quantitative assessment of most clinical samples.     

Determination of free and total cyst(e)ine in plasma of dogs and cats

BACKGROUND: In human blood, the amino acid cysteine forms disulfide bonds with itself and with other sulfhydryl compounds in their free form and with sulfhydryls in protein. Protein-bound cysteine is lost when plasma proteins are removed before amino acid analysis. OBJECTIVE: The purpose of this study was to assess the time course and extent of cyst(e)ine (cysteine + half-cystine) loss in dog and cat plasma. METHODS: An equal volume of 6% sulfosalicylic acid was added to plasma aliquots at 0, 2, 4, 10, 16, 24, 36, 48, 60, and 72 hours after separation of blood cells. Tris-2-carboxyethyl-phosphine hydrochloride (TCEP - HCl), a reducing agent, was used to regenerate total plasma cyst(e)ine after 3 months of sample storage (-20 degrees C). RESULTS: Initial free cyst(e)ine concentrations (mean +/- SEM) were higher in canine plasma (77 +/- 4 micromol/L) than in feline plasma (37 +/- 3 micromol/L). Free plasma cyst(e)ine concentrations in dogs and cats decreased after first-order kinetics, with a half-life of 23 and 69 hours, respectively. Total plasma cysteine after TCEP - HCl treatment was similar for dogs (290 micromol/L) and cats (296 micromol/L), but the percentage of free cysteine was higher (P = .02) in dogs (27%) than in cats (13%). Over half of the cyst(e)ine, homocysteine, cysteinylglycine, and glutathione were bound in vivo to plasma proteins. CONCLUSION: These results emphasize the importance of removing plasma proteins within 1 hour after blood collection for reliable assay of free plasma cyst(e)ine.     

Correlation between fine-needle aspiration cytology and histopathology in the evaluation of cutaneous and subcutaneous masses from dogs and cats

BACKGROUND: Fine-needle aspiration cytology (FNAC) is commonly used as a diagnostic procedure to evaluate superficial and deep masses in animals. However, few studies have addressed the accuracy of FNAC in the evaluation of cutaneous and subcutaneous masses in a clinical setting. OBJECTIVE: The purpose of this study was to compare the accuracy of FNAC as compared with histopathology in the diagnosis of cutaneous and subcutaneous masses from dogs and cats. METHODS: Cytologic and histopathologic specimens obtained between 1999 and 2003 from 292 palpable cutaneous and subcutaneous masses obtained from 242 dogs and 50 cats were retrospectively evaluated. Cytologic samples were obtained by FNA and histopathologic samples were collected by surgical biopsy or at necropsy. Concordance was determined and the accuracy of FNAC for the diagnosis of neoplasia was determined using histopathology as the gold standard. RESULTS: Of 292 specimens, 49 (from 44 dogs and 5 cats) were excluded due to poor cellularity of the cytologic specimen (retrieval rate 83.2%, n = 243). A cytologic diagnosis of neoplasia was obtained in 176 cases (175 true positives and 1 false positive compared with histopathology). Sixty-seven cytology samples were classified as non-neoplastic (46 true negatives, 21 false negatives compared with histopathology). Overall, the cytologic diagnosis was in agreement with the histopathologic diagnosis in 90.9% (221/243) of cases. For diagnosing neoplasia, cytology had a sensitivity of 89.3%, a specificity of 97.9%, a positive predictive value of 99.4%, and a negative predictive value of 68.7%. CONCLUSIONS: The results of this study confirmed FNAC as a reliable and useful diagnostic procedure for the evaluation of palpable cutaneous and subcutaneous lesions in small animal practice.     

Evaluation of urine specific gravity and urine sediment as risk factors for urinary tract infections in cats

Background: It has been suggested that diseases that promote isosthenuria predispose to urinary tract infections because of a lack of the common bacteriostatic properties present in concentrated urine. Objectives: The purpose of this study was to assess the clinicopathologic risk factors for positive urine culture outcome in cats with chronic kidney disease (CKD), diabetes mellitus (DM), uncontrolled hyperthyroidism (HT), or lower urinary tract disease (LUTD). Methods: For this retrospective study, medical records of all cats in which a urinalysis and aerobic bacterial urine culture were performed between January 1995 and December 2002 were reviewed. Signalment, body weight, and clinicopathologic data were recorded. Based on the medical records, cats were diagnosed with CKD, DM, HT, or LUTD. Prevalence odds ratios and 95% confidence intervals were calculated using logistic regression. Multivariate models were created for each variable of interest while controlling for the confounding effect of disease group. Results: Six hundred fourteen cats met the criteria for inclusion in the study. Overall, positive urine cultures were identified in 16.9% of cats with CKD, 13.2% of cats with DM, 21.7% of cats with HT, and 4.9% of cats with clinical signs of LUTD. Decreasing urine specific gravity was not associated with positive urine culture when controlled for disease but pyuria, bacteriuria, and hematuria were all associated with positive urine culture outcome. Persians, females, increasing age, and decreasing body weight were all associated with positive urine culture outcome. Conclusions: Performing a urine culture sample based solely on the presence of isosthenuria does not seem warranted. Further studies are warranted to help identify host predisposing factors for urinary bacterial colonization in cats with these diseases.     

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

Abstract: 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.     

Comparison of semiquantitative test strips,urine protein electrophoresis,and an immunoturbidimetric assay for measuring microalbuminuria in dogs

Background: The presence of albumin in urine, even in small amounts, is always abnormal and usually reflects kidney dysfunction. Different techniques are commercially available for the measurement of microalbuminuria in dogs. Objectives: The purpose of this study was to compare the accuracy of semiquantitative test strips, urine protein electrophoresis, and a validated immunoturbidimetric assay in the measurement of microalbuminuria in dogs. Methods: Urine samples were collected from 307 dogs presented to The Queen's Veterinary School Hospital, University of Cambridge, for a variety of clinical conditions. Urine was collected by midstream free catch (193/307, 63%), cystocentesis (89/307, 29%), or catheterization (25/307, 8%). Routine urinalysis was performed on all samples. Albumin was measured by using semiquantitative test strips, by agarose gel electrophoresis, and by an automated immunoturbidimetric assay designed for human samples (considered as the gold standard). The latter was validated using a purified canine albumin standard. Results: The immunoturbidimetric assay had within‐assay and between‐assay coefficients of variation (CV) of 1.3% and 5.0%, respectively, overall recovery of 97.1%, and high linearity (r=.985). Of the samples with measurable albumin (>1.4 mg/L) by the immunoturbidimetric assay, 57/195 (29%) were negative for albumin using the semiquantitative test strips and 138/195 (71%) were positive. Urine protein electrophoresis (UPE) and immunoturbidimetric results had a concordance CV of 86%. Conclusions: UPE and semiquantitative test strips are less accurate than the automated immunoturbidimetric method for the measurement of albumin in canine urine.