Discordance between urine pH measured by dipstick and pH meter: Implications for methotrexate administration protocols

Analytical performance evaluation of two automated urine chemistry analysers using two levels of commercially available quality control material

OBJECTIVE

Evaluate two point-of-care urine chemistry analysers, VetScan SA and VetLab UA using assayed, bilevel (two concentrations) urine quality control material to determine if performance is acceptable for semiquantitative clinical urine chemistry analysis.

MATERIALS AND METHODS

Normal and abnormal urine quality control material sent to 23 veterinary practices was evaluated three times by each clinic on in-clinic automated urinalysis instruments. Accuracy, precision and clinical utility were evaluated.

RESULTS

Normal urine quality control material: Results for blood, glucose, ketones and bilirubin were 100% accurate and precise for both analysers, and pH values were accurately acidic to neutral. However, pH from VetScan SA had clinically significant negative bias. Abnormal urine quality control material: VetScan SA: blood, microalbumin and bilirubin were 100% accurate; glucose, ketones, and protein demonstrated ≤10% inaccuracy; pH demonstrated 34% inaccuracy. VetLab UA: blood, ketones and bilirubin were 100% accurate; glucose and protein demonstrated ≤10% inaccuracy; pH was 100% accurately neutral to alkaline.

CLINICAL SIGNIFICANCE

VetScan SA had marked negative pH bias versus VetLab UA resulting in clinically significant, overly acidic results. Specific gravity, nitrite, and leukocyte test pads should not be used. Both instruments had excellent performance in normal quality control material. While blood, glucose, protein and bilirubin are correctly identified as present in abnormal quality control material, exact concentrations cannot be interpreted due to imprecision. Only semiquantitative results, not numerical values implying quantification, should be reported from urine test strips.

Evaluation of urinary acidification in children: Clinical utility

The kidney plays a fundamental role in acid-base homeostasis by reabsorbing the filtered bicarbonate and by generating new bicarbonate, to replace that consumed in the buffering of non-volatile acids, a process that leads to the acidification of urine and the excretion of ammonium (NH₄ ⁺). Therefore, urine pH (UpH) and urinary NH₄ ⁺ (UNH₄ ⁺) are valuable parameters to assess urinary acidification. The adaptation of automated plasma NH₄ ⁺ quantification methods to measure UNH₄ ⁺ has proven to be an accurate and feasible technique, with diverse potential indications in clinical practice. Recently, reference values for spot urine NH₄ ⁺/creatinine ratio in children have been published. UpH and UNH₄ ⁺, aside from their classical application in the study of metabolic acidosis, have shown to be useful in the identification of incomplete distal renal tubular acidosis (dRTA), an acidification disorder, without overt metabolic acidosis, extensively described in adults, and barely known in children, in whom it has been found to be associated to hypocitraturia, congenital kidney abnormalities and growth impairment. In addition, a low UNH₄ ⁺ in chronic kidney disease (CKD) is a risk factor for glomerular filtration decay and mortality in adults, even in the absence of overt metabolic acidosis. We here emphasize on the need of measuring UpH and UNH₄ ⁺ in pediatric population, establishing reference values, as well as exploring their application in metabolic acidosis, CKD and disorders associated with incomplete dRTA, including growth retardation of unknown cause.

Introduction of a Standardized Approach of Electronic Urinary pH Monitoring to Assist Alkalization Therapy: A Uric Acid Urolithiasis Patient's Perspective

Introduction: This study evaluates the introduction of an electronic pH meter to measure the urinary pH in patients with uric acid (UA) urolithiasis and assess patient's perspective. Materials and Methods: Patients known with UA urolithiasis were included in this single-center, nonrandomized, prospective feasibility study, IDEAL stage 2a. Their experience with urolithiasis and satisfaction with the method of urinary pH monitoring before inclusion was evaluated. All patients received an electronic pH meter and standardized instructions. After a period of 6-12 weeks their experience and satisfaction with this pH meter and new regimen was assessed. Patient satisfaction was scored on a Likert scale 1-5. Results: Eighteen patients were included. Median age was 63 years and median body mass index was 30 kg/m². The cohort consisted of 67% men and 33% women. In their medical history, 55% had unilateral stones, whereas 45% had bilateral stones. The median estimated glomerular filtration rate was 58 mL/minute/1.73 m². Eighty-nine percent took medication to alkalize their urine, median 3.5 years. Fifteen patients used paper reagent strips and three used an electronic pH meter to assess urinary pH before this study. Satisfaction with the method of urinary pH measurement at inclusion was reasonable (median score 3; interquartile range [IQR] 1-4). Satisfaction with the new electronic pH meter was good (median score 4; IQR 3-5), as was the overall satisfaction (median score 4; IQR 3-5). The new electronic pH meter was slightly easier to use (median 3.5; IQR 1.75-5), as easy in maintenance (median 3; IQR 2-4), and significantly easier to read (median 5; IQR 4-5). The new electronic pH meter was better (median score 4; IQR 2.75-5) than their previous method. Conclusion: The introduction of a standardized approach of urinary pH monitoring for UA urolithiasis patients with an electronic pH meter leads to an easier interpretable outcome and higher patient satisfaction.

Optimizing the u411 automated urinalysis instrument for veterinary use

BACKGROUND

The Cobas u411 Analyzer (Roche Diagnostics) is an automated, reflectance photometry-based urinalysis instrument designed for use with Roche's CHEMSTRIP 10UA technology and human urine samples.

OBJECTIVE

We aimed to optimize and validate the Cobas u411 Analyzer for use in canine and feline urinalysis.

METHODS

Patient urine samples presenting to the Clinical Pathology Laboratory at the Colorado State University Veterinary Teaching Hospital were analyzed with the Cobas u411 and by manual readings in parallel. Initially, 223 canine and 83 feline urine samples were run using the u411 factory settings. Following comparisons with manual results, and evaluation for directional bias, adjustments to the reflectance values were made in the instrument's programming. An additional 183 canine and 95 feline samples were run using the adjusted settings. Total urine protein concentrations were measured in 48 samples and used to generate receiver operating characteristic curves for the protein test pad.

RESULTS

Following adjustments in reflectance programming, concordance between u411 and manual results was increased by 17.7% for protein, 11.7% for ketones, and 4.5% for bilirubin. Concordances for pH, glucose, and blood were not substantially changed. Discordance for all analytes was ≤3%. Canine and feline samples had similar levels of discordance, though marginal concordance was higher in dogs for ketones, bilirubin, and blood.

CONCLUSIONS

Adjustments to the reflectance programming of the Cobas u411 Analyzer improved concordance with manual results for canine and feline samples. This instrument has the potential to greatly increase both efficiency and consistency of urinalysis procedures in higher throughput veterinary diagnostic laboratories.

2,4-Diamino-N10-methylpteroic acid (DAMPA) crystalluria in a patient with osteosarcoma treated with carboxypeptidase-G2 rescue after high-dose methotrexate-induced nephrotoxicity

BACKGROUND

High-dose methotrexate (HDMTX) therapy is a key component of many chemotherapy protocols. However, some patients develop HDMTX-induced nephrotoxicity. Carboxypeptidase-G2 (CPDG2) hydrolyses MTX into 2,4-diamino-N10-methylpteroic acid (DAMPA) and glutamic acid, and is used as a rescue agent in patients with nephrotoxicity and delayed elimination. Despite the frequency of HDMTX-induced renal injury, crystalluria is uncommon. Furthermore, crystals are rarely identified by conventional chemical methods.

OBJECTIVE

To determine the composition of crystalluria in a patient with osteosarcoma who was treated with CPDG2.

METHODS

Crystalluria was evaluated by optical microscopy, and chemical identification was performed by Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and Orbitrap™ high-resolution mass spectrometry (HRMS).

RESULTS

The HRMS spectra of the patient's urine sediment showed a main peak at m/z 326.13, corresponding to the molecular mass of DAMPA [(C₁₅H₁₅O₂N₇) + H⁺]. The FT-IR spectral patterns of the sediment and DAMPA were not identical. SEM was unable to identify the crystal.

CONCLUSION

DAMPA crystalluria was identified by Orbitrap™ HRMS in a patient treated with CPDG2 after HDMTX nephrotoxicity. This case reinforces the need to implement adequate measures to prevent nephrotoxicity. In cases of HDMTX-induced nephrotoxicity, urine sediment analysis should be requested.