The authors respond:
We wish to thank Dr Heeley for his comments regarding measurement of the urine protein:creatinine ratio (UPC). We believe our comments to be correct in the context they were made, namely that standard chemistry analysers cannot perform UPC. The UPC test by IDEXX analysers (Catalyst and VetTest) is a specific test and not a standard test: it does not use the routine protein and creatinine reagents for serum in the measurement for urine testing, which, as outlined in our article,
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are problematic.
Regardless, we sought information from IDEXX as to how the IDEXX UPC test had been validated and whether there had been independent validation performed. UPCs should always be interpreted with care as they can differ, in dogs at least, with different sample handling and different laboratories.
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The IDEXX guidelines for its test appear to reflect International Renal Interest Society (IRIS) guidelines. The IRIS guidelines provide no information as to which methodology was used, albeit they appear to be based on studies that used wet chemistry for UPC measurement.
Dr Graham Swinney from IDEXX (Sydney) kindly supplied the company’s validation data for this test (see supplementary material). It was not possible for him to supply the raw data as requested and the validation data assessed samples from both cats and dogs. The information provided by IDEXX shows an excellent correlation between the IDEXX UPC as run on VetTest and the reference methodology. It is perhaps of some concern that the comparison was to a dry chemistry ‘reference’ method. The values observed with dry chemistry systems are often different from those produced by traditional wet chemistry methods, and it would have been more useful if IDEXX had compared its test to a wet chemistry method in addition to a dry chemistry method. As it stands, the comparison would appear to be between the same slides on two versions of the dry chemistry systems; thus it would have been surprising to see anything other than a near-perfect correlation. Regardless, excellent correlation (a linear association) does not necessarily compute to excellent agreement of measurements. Despite some confusing wording in the document, the IDEXX UPC document contains no information about agreement between the two methods.
We were surprised to note some UPCs as high as 14 in cats in the IDEXX graph as this would be unusual on wet chemistry analysers. It is possible that the dry chemistry UPC reads higher than wet chemistry UPC due to poor agreement with wet chemistry measurements. At Vetnostics (Sydney, Australia) we have assessed one canine case in which the IDEXX Catalyst UPC was 4.69 while the laboratory result was 2.3. The sample was then sent to IDEXX Laboratory and its result was 2.4, indicating excellent agreement between the two wet chemistry analysers used by both commercial laboratories. The in-house urine protein appeared to be overestimated in this particular dog. A single case is inadequate to assess methodology but the result is concerning nonetheless given the tight IRIS guidelines.
As such, until independent validation of the IDEXX UPC is performed, the authors would consider the IDEXX UPC test to be a preliminary screening test only and would be hesitant to recommend the IDEXX UPC test as a basis for treatment decisions.
Sue Foster BVSc, MVetClinStud, FANZCVS
George Reppas BVSc, DipVetPath, FANZCVS, Dipl ECVP
Supplemental Material
Click here for Supplementary Material
IDEXX Urine P:C Ratio Method Comparison Study
Footnotes
Supplementary material: IDEXX Urine P:C Ratio Method Comparison Study; 2004, IDEXX Laboratories.
References
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1.
Reppas G, Foster SF. Practical urinalysis in the cat. 1: Urine macroscopic examination ‘tips and traps’. J Feline Med Surg
2016; 18: 190–202.
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