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From |
guhjy@kmu.edu.tw |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model? |

Date |
Mon, 28 May 2012 00:17:28 +0800 |

In general, your comments are correct (esp. for an individual patient). However, low urinary concentration is an independent predictor of CKD progression (http://www.ncbi.nlm.nih.gov/pubmed/12722030?dopt=Abstract) and there has been some suggestions that the analyte concentration (unadjusted for creatinine) should be included in the analysis with urinary creatinine added as a separate independent variable in a population of groups (http://www.ncbi.nlm.nih.gov/pubmed/15687057). Jinn-Yuh 2012/5/27 Phil Clayton <philclayton@internode.on.net>: > On 27/05/2012, at 9:35 PM, David Hoaglin wrote: > >> As an explanatory variable, ACR is one function >> of urinary albumin and urinary creatinine; but you could reasonably >> consider other functions, such as the linear combination of urinary >> albumin and urinary creatinine that arises from using those two as >> explanatory variables or the nonlinear function in which the >> explanatory variables in that part of the model are urinary albumin, >> urinary creatinine, and their product > > What we're interested in, biologically, is the 24-h urinary albumin excretion. > > The reason the albumin is divided by creatinine is that the ACR is used to estimate the 24-h urinary albumin excretion from a single urine specimen rather than asking someone to collect their urine for a day. The urinary concentration can vary several fold (eg if you're dehydrated it goes up) which changes the albumin concentration in that specimen - but it changes the creatinine concentration by a similar amount, and we know the normal 24-h excretion of creatinine, so we divide the albumin by the creatinine to estimate the 24-h albumin excretion. > > So biologically ACR should be a ratio and must be nonnegative. You could include it in a regression model as a surrogate for the 24-h urine albumin excretion, but would need to careful how to model it as it generally has a nonlinear effect. For example it is commonly modelled as a categorical variable - normal, microalbuminuria, macroalbuminuria. > > Phil > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*Phil Clayton <philclayton@internode.on.net>

**References**:**st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*guhjy@kmu.edu.tw

**Re: st: Is it valid to use the individual ratios (i.e. Xi/Yi) in the dependent or independent part of a regression model?***From:*Nick Cox <njcoxstata@gmail.com>

*From:*Steve Samuels <sjsamuels@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*Tirthankar Chakravarty <tirthankar.chakravarty@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*Nick Cox <njcoxstata@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*David Hoaglin <dchoaglin@gmail.com>

*From:*guhjy@kmu.edu.tw

*From:*David Hoaglin <dchoaglin@gmail.com>

*From:*Phil Clayton <philclayton@internode.on.net>

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