I am working on a review of membranous nephropathy and I found this quotation from UpToDate:
A random urine protein-to-creatinine ratio should NOT be used for initial risk stratification, since the relationship between the ratio and 24-hour protein excretion varies widely among patients (show figure 2).
This surprised me. I use the protein-to-creatinine ratio all the time and, though I have found some individuals where it is wildly inaccurate, I had been unaware of any data that showed that to be the case.
I eagerly clicked the link to the figure and this is what I found:
That looks highly accurate to me. Every data point clusters right along the line of identity. I pulled the abstract (alas, no full text from NEJM 1983) and found this conclusions from the authors:
In a study of 46 specimens we found an excellent correlation between the protein content of a 24-hour urine collection and the protein/creatinine ratio in a single urine sample.
I love UpToDate but this is really disappointing. Making a claim and referencing it with data which disproves the claim is disingenuous.
UpToDate, you need to do better.
If you look a the figure then you see that at a P/C ratio of 4 the 24 h P excretion could be either 2 or 8 grams or anything in between. I don't think that is very accurate.
@Rene I think the point is that the experimental data actually when considered together as a sample correlate well- obviously there are outliers but they are statistically insignificant
But I wonder whether their 24h protein excretions were accurate? I doubt the ones my patients bring are.
Also wonder how many of us ever corrected 24h protein excretion for body size (or creat excretion) anyway? We know we should.
Is there a study that compares PCR in a spot sample versus PCR in a 24h sample?
The figure is not ideal because there are so few cases of patients with nephrotic range proteinuria. It looks like 80% of the data is from non-nephrotic patients. In membranous we really need to differentiate 6 grams from 10 grams, it can move a patient from moderate to high risk and change the treatment plan.
I agree with the sentiment of UpToDate that you need a 24 hour collection, I was so upset because they used this data to support that conclusion but this data at best is not ideal to answer question and just a little less charitably proves the opposite point.
Up To Date is generally a great resource–but as this post suggests, it is not perfect, and sometimes there is no substitute for going back to the original literature and evaluating for oneself!
so true!