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I'm using siunitx to handle my units for some notes I'm writing but am having a bit of trouble with defining units for glomerular filtration rate (a measure of kidney function). This is measured in ml/min/1.73m^2 for historical reasons, which I want to render as ml·min·1.73m^-2. I have attempted to define this in the MWE below.

\documentclass[10pt,a4paper]{book}

\usepackage[inter-unit-product=\ensuremath{{}\cdot{}}]{siunitx}

\newcommand{\mlkh}[1]{\SI{#1}{\milli\litre\per\kilo\gram\per\hour}} % for e.g. urine output
\newcommand{\gfr}[1]{\SI{#1}{\milli\litre\per\minute\per\text{1.73}\metre\squared}} % for eGFR

\begin{document}

The patient had a urine output of \mlkh{1.0}.

His GFR was estimated at \gfr{90}.

\end{document}

Output:

enter image description here

Here the first command (\mlkh) works as I expect and gives output with reciprocal units and centre dots but the second command (\gfr) uses slashes to separate the units and is not reciprocal.

Can anyone point out what I'm doing wrong? Is it the presence of the \text{1.73} which is problematic or am I doing something more basic wrong?

Many thanks.

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  • 1
    Not a nice solution, but it works: \newcommand{\gfr}[1]{\SI{#1}{\milli\litre\cdot\si{\per\minute}\cdot\SI{1.73}{\per\metre\squared}}}.
    – Benjamin
    Commented Dec 16, 2015 at 11:16
  • 1
    I'm not sure (1.73m^2)^{-1}=1.73m^{-2}; am I missing something?
    – egreg
    Commented Dec 16, 2015 at 11:25
  • @egreg -- Yes, now I think about it you're probably right. Sadly doctors are fairly blasé about things like 'correctly using mathematics' and this convention has stuck to the point that it appears in Nature so I'm loathe to change it.
    – JWC
    Commented Dec 16, 2015 at 23:07
  • @Benjamin -- Thanks. If it works I don't care if it isn't pretty.
    – JWC
    Commented Dec 16, 2015 at 23:08

1 Answer 1

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I have no problem if using \sisetup:

\documentclass[10pt,a4paper]{book}

\usepackage{siunitx}
\sisetup{inter-unit-product=\ensuremath{{}\cdot{}}}

\DeclareSIUnit{\strange}{\num{1.73}\,m}

\newcommand{\mlkh}[1]{\SI{#1}{\milli\litre\per\kilo\gram\per\hour}} % for e.g. urine output
\newcommand{\gfr}[1]{\SI{#1}{\milli\litre\per\minute\per\strange\squared}} % for eGFR

\begin{document}

The patient had a urine output of \mlkh{1.0}.

His GFR was estimated at \gfr{90}.

\end{document}

enter image description here

Give \strange a better name. ;-)

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