I'm rather puzzled about the 20% retention rate following 4 ml 50% MgSO4
IM in the buttocks (that sounds very painful too!). Are you able to
provide the reference to this?
The referential data I've got refers to an intravenous infusion of 30
mmol MgSO4 in 1 litre of isotonic saline over 8 hours.
The 24 hr magnesium retention rate in normal subjects was reported to
range from -19.5% to 27.5% (Scand J Clin Lab Invest 1994;54:23-31).
And if this loading test is repeated on consecutive days to identify
magnesium deficiency (Crit Care Med 1997;25:749-55), any possible
deficiency is automatically corrected! We don't measure urinary
magnesium for our ITU patients now!
Chan Seem
King's Lynn
-----Original Message-----
From: Nick Miller [SMTP:[log in to unmask]]
Sent: 30 July 2000 23:27
To: [log in to unmask]
Subject: Low magnesium - what does it mean
Vivekanadan Sachidanandam wrote:
"Regarding the best method of body Magnesium status,
scientifically,
I feel measuring the urine Mg before and after Mg bolus
reflecting the
cellular uptake in a subject with a normal renal function, is
scientifically
sound and adopted by many clinical drug trial studies."
Agreed. The test suffers from all the problems of 24 hour urine
collections but is otherwise the baseline test for magnesium de-
saturation. The protocol is to collect a 24 hour urine for magnesium
measurement and then give 2 ml of 50% MgSO4 deep IM in each buttock (8
mmoles of magnesium altogether). You then collect a second 24 hour
urinary magnesium and calculate the percentage of magnesium retained.
While normal subjects excrete around 80% of the load, those who retain
more than 20% are regarded as magnesium deficient (provided they are
not suffering from renal insufficiency).
The fact that some textbooks say that magnesium deficiency
rarely, if ever, occurs in human subjects, just suggests to me that the
authors never actually looked for this problem ! There are plenty of
people these days eating diets that do not adequately replace their
magnesium losses and they become progressively deficient, with a
variety symptoms that are often not associated with electrolyte
problems.
Whereas it may be fairly easy to predict Mg requirement in the
in- patient situations that Lars Beirmer mentions, such measurements
are important in chronically sick outpatients, some of whom may be
magnesium deficient, others not.
Measuring serum magesium, as Rick Jones points out, is of no
help in this situation.
Nick Miller
London
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