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I am trying to calculate relative organ weight for a rat. Anyone knows of a proper formula. It needs to be expressed in g/kg BW.

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    $\begingroup$ Do you have access to the rat and the organs? $\endgroup$ – kmm Nov 23 '17 at 0:06
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Basically the formula is:

$$w_r=\frac{w_{organ}}{w_{animal}}$$

Nominally this would be in g/g (since given the weight ranges for both rats and their organs, g is the most adequate unit, and the one you will likely be measuring in on your scales). If you want to force the g/kg unit, you would simply multiply by 1000:

$$w_r=1000\frac{w_{organ}}{w_{animal}}$$

If you have the weight of the organ, you would simply need to find out the weight of the rat:

  • Ideally this would be measured before the rat is killed.
  • If you have already extracted the organ, the weight of the rat would be determined from the sum of the weight of the organ and the weight of rest of the cadaver. This is, of course, assuming that the amount of blood and interstitial tissue lost during the operation is negligeable and that the animal was not perfused before organ extraction. The latter asssumption may well be met, but the first one in all likelihood is not. So this estimate will be biased (towards a slightly higher value) but it may be the best you can do.
  • If the cadaver has already been discarded you can estimate the mean weight a rat of the same strain at the same age (measure a number of them and determine the mean), and use that value. This will be an even more unreliable estimate, and depending on your treatment (i.e. if you are manipulating the rat weight and not the organ weight) perhaps even worthless, but again, it may be the best you can do.
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suppose mice weight is 35 gm and its liver weight is 1.5 gm then relative organ weight would be ;

per 35 gm body weight, liver weight is 1.5 gm Then, 100 gm body weight, liver weight is?

Therefore= 100*1.5/35 = 4.29 gm liver weight/100 gm body weight

means 4.29% of body weight is because of liver. You can make formula in Excel and can carry out big data calculation at ease.

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  • $\begingroup$ I suppose the OP knows about percentages. Moreover, the question explicitly asks that the relative organ weight has to be expressed in g/kg. Can you edit as to include it? $\endgroup$ – LinuxBlanket Feb 26 '18 at 9:57

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