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Date: | Wed, 30 Apr 2008 10:38:59 -0400 |
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I liked Gonneke's comment that suggested common sense will give us the answer for
some things without needing a full scale study.
Every now and again I am struck by the question of "where did that procedure come
from".
So, here's my questions on both common sense and on evidence grounds:
Common sense question: If you have a more direct indicator that is readily at your
disposal (e.g. test weighing a baby before and after a feeding) compared to an indirect
untested indicator (e.g. test weighing a diaper), why would you ever choose the indirect
indicator over the direct indicator?
Evidence based question: Is there any evidence that shows that test weighing a diaper
has adequate sensitivity and specificity to be useful as an indicator of intake? If number
of diapers does not have good sensitivity and specificity, I don't see how tiny little
increments in mass are going to be any better. I'd love to see some studies on this to
see what the ROC curves are like.
Then, digging further into this issue, I see two epidemiologically based problems with test
weighing diapers:
1) Measurement error: when you measure a lighter object that is closer to the limits of
accuracy --- the percentage of measurement error compared to the weight of the object
increases. This means that if you weigh a baby which weighs more than a diaper --- you
are likely to get a lower proportion of measurement error.
2) Confounding factors: when you are measuring output as a proxy indicator for input,
there are many factors that could confound the situation. For instance, IV fluids or kidney
function.
So, it seems to me that weighing a diaper might have been useful in the situation when I
worked with a baby that was ultimately diagnosed as having a metabolic disorder who
was drinking plenty (3-4 ounces 8-10x/day) but rapidly losing weight. But I really can't
quite figure out why you would choose the indirect over the direct measurement of intake
if intake is what you want to measure.
Best, Susan
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