If the plasma urea is greatly raised and the creatinine normal or only
marginally raised, there is little doubt that a patient has either prerenal
renal failure (dehydration, congestive failure) or an increased urea load (GI
haemorrhage) rather than glomerular failure. I suspect that, of the causes of
prerenal failure, only mild dehydration is likely to be missed clinically,
but measuring both analytes is a cheap way of  monitoring progress.

Jeff Slater mentioned the urea:creatinine ratio. This seemed to come into use
in the early 1970s more on the basis of physiological principle than of
clinical evidence. For this reason in 1977 Brian Morgan, Margaret Carver and
I examined the case notes of 131 patients who had both measurements and who
had a urea >10mmol/L so that we could relate the ratio to diagnosis (BMJ
1977; 2: 929-32). We found that we were less able to distinguish prerenal
from intrinsic renal failure using the ratio than using  the plasma
creatinine concentration alone because a wide range of ratios was possible
when the measurements were not greatly raised. When the urea concentration
was high, the ratio distinguished the two groups, but then so did the
creatinine concentration. In our patients a creatinine >250umol/L indicated
intrinsic renal failure with a 90% probability.

Brian Payne

Jeff Slater wrote:

> I don't understand this debate re: creatinine and urea. I suggest that
> it may be perceived as an index of demand management and if you measure
> both your Head of Department is a useless old fogey.
> In days long gone (i.e. continuous-flow)  a UE was urea and
> electrolytes,  we felt that we could measure urea better than
> creatinine, creatinine was better reserved for clearances but we
> measured both anyway.
> Then along came discrete analysers and expensive enzymatic urea methods
> so we negotiated with the renal physicians and only did ureas on
> creatinines >150 umol/L - even though the precision on creatinines was
> lousy and they were subject to interference by bilirubin and ketone
> bodies.
> Then it started to dawn on us that we were missing patients who were
> dehydrated e.g. little old ladies with chest infections, apparently
> normal creatinines with ureas (not measured) of 20 mmol/L.
> So the policy became both on all urgents, all A&E, all ITU, all Burns
> ITU, all renal unit etc. and we still got complaints from junior doctors
> who could not understand why they didn't get a urea when they asked for
> a UE stupid naive fools.
> Well now I'm short-staffed, the stressed operators were missing ureas on
> the patients who would benefit from them, 30-40% were getting them
> routinely anyway and the extra marginal cost of doing ureas on everyone
> is < £5k so as of earlier this year everybody gets both.
> However I also regularly talk to the junior medics about urea/creatinine
> ratios and their relevance to dehyradation and I also talk about the
> separate interpretation of creatinine and urea values via tutorials.
> I've investigated reference ranges more thoroughly and use male and sex-
> dependent ranges.
> so there
>
> In message <[log in to unmask]>,
> Elizabeth Mac Namara <[log in to unmask]> writes
> >    I am not quite sure where the impetus for the omission of urea
> >    comes from. In Quebec the Ministry of Health now sends us all
> >    messages with our yearly statistics as to what percentage of
> >    patients get the two tests and how we compare to the rest of the
> >    province in this ratio. I have been even ask to respond in writing
> >    as to why I do not stop physicians from ordering the two together.
> >    I think the combination is a particularly good one especially for
> >    all hospitalised patients and all patients taking Lasix or other
> >    diuretic. Older patients particularly are at risk of dehydration. I
> >    always look at the two results when I work as an  internist on one
> >    of the acute medical wards or as part of nutrition rounds. I would
> >    be interested to know why so many people feel it is futile.
> >    Certainly well healthy adults may not need it but I would forgo
> >    many other tests before that combination.
> >
> >    Elizabeth Mac Namara
> >    Dept of Diagnostic Medicine
> >    Jewish General Hospital
> >>       -----Original Message-----
> >>       From: clinical biochemistry discussion list
> >>       [mailto:[log in to unmask]]On Behalf Of Robin
> >>       Marks
> >>       Sent: July 24, 2002 12:28 PM
> >>       To: [log in to unmask]
> >>       Subject: creatinine and not urea
> >
> >
> >>       Hello all !
> >
> >
> >>       I would be interested to know if any laboratories have tried
> >>       stopping measuring urea in favour of creatinine, as part of a
> >>       larger panel ('urea and electrolytes') or otherwise.
> >
> >
> >>       I would be interested to hear of any positive of negative
> >>       experiences.
> >
> >
> >>       Dr Robin Marks
> >>       Consultant Chemical Pathologist
> >>       Calderdale Royal Hospital
> >>       Halifax
> >>       HX3 0PW
> >>       UK
> >
> >>       ------ACB discussion List Information-------- This is an open
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> >>       in clinical biochemistry. Please note, archived messages are
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> >>       those of the individual and they are responsible for all message
> >>       content.
> >
> >>       ACB Web Site http://www.acb.org.uk List Archives
> >>       http://www.jiscmail.ac.uk/lists/ACB-CLIN-CHEM-GEN.html List
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> >
> >    ------ACB discussion List Information-------- This is an open
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> >    and can be viewed via the internet. Views expressed are those of
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> >    ACB Web Site http://www.acb.org.uk List Archives
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> --
> Jeff Slater
>
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