Annadurarai,

Are you measuring fasted blood?Here is an article that corroborates the fact that both rat and human triglyceride levels are raised by dietary excess, particularly fat and sugar.  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3522469/pdf/nihms189182.pdf Perhaps a measurement of total cholesterol, triglycerides, and HDL would be a better indicator than using LDL. Also there is non-HDL cholesterol which is total cholesterol minus HDL. This give an account of triglycerides, apo a, very-low density lipoprotein cholesterol and LDL all combined and is considered the cholesterol total that contributes to atherosclerosis. Atherosclerosis type cholesterol is measured by non-HDL cholesterol.

First, the rat VLDL does not have the same structure as the human, the Friedewald formula described for human and has the disadvantage that can not be used if there are changes in VLDL (as hipertrigliceridcemia). In my opinion, the only way to obtain the measurement of LDL by UC.

An alternative is to use the magnitude called "non-HDL cholesterol"; is not parallel to the LDL, but also we have to think of the rat lipoproteins are not as human.

If you dont have acces to an UC, is better to use the profile: Triglycerides, Cholesterol, HDL cholesterol and non-HDL Cholesterol (Total cholesterol-HDL cholesterol).

I am assuming that the Friedewald formula will be regardless of human or rat plasma. I therefore think that the upper limit of 400 mg/dl should be used. In humans our lab has been using direct measurements of LDL cholesterol for several years

I agree with Juan Gerique. At the same time, in human medicine, Friedewald formula ceased to be sufficiently valid after the publication of Martin et al formula (Martin et al: Comparison of a novel method vs. the Friedewald equation for estimating low-density lipoprotein cholesterol levels from the standard lipid profile. JAMA, 2013 )

The Friedewald equation is the following formula: LDL-c = [total cholesterol - HDL-c] - triglycerides / 5 The result is expressed in milligrams per deciliter. "The equation applies a factor of 5 to everyone, which is not fair," the authors express. "A more appropriate formula should take into account the specifics of cholesterol and triglycerides individual. "

is better to use the profile: Triglycerides, Cholesterol, HDL cholesterol and non-HDL Cholesterol (Total cholesterol-HDL cholesterol).

Slimen, you are right.

Martin´s formula (Martin et al, JAMA, 2013) present a novel method

for estimating LDL-C from the standard lipid profile. Rather

than assuming a fixed factor of 5, it applies an adjustable factor

for the TG:VLDL-C ratio based on triglyceride and non–

HDL-C concentrations in the 180-cell approach. Compared with Friedewald

estimation, classifications based on US and European

clinical practice guidelines using LDL-C estimates by the novel

method are more concordant with those by LDL-C measured with gold standard method. (But we are slightly deviated from the original question on rats.)

Cut off point fot triglycerides in human is 150 mg/dL. In rats we must know  if they are Wistar or Sprague Dowley Do not use formulas , but made measurments of TG, LDL-Chol and HDL chol. Rats do not have CETP

As Dr Gomez answered, Friedewald formula should not be applied to rodents. Ultracentrigation or FPLC may be used for determining LDLc or, alternatively, determine HDL by precipitating apoB-containing lipoproteins and measure non HDLc

The agreement is overwheliming. Previous studies in our laboratories demosstrate that the Friedewaid formula is simply useless in humans. Not to mention in rats. Either non-HDL cholesterol or FPLC assessment are acceptable in rodents. If interested, follow a related debate: Angiology. 2014 Mar;65(3):261-2.

We do not use Friedewald formula-  Many years ago,  Friedewald in person answer  my question  about his formula, and told me  that the value TG/Col=5 is useful for typical native VLDL in humans with Plasma TG lower than 400 mg/dL. A recent paper propose the ratioTG/ Col in humans = 5.2. . As Jorge says, Friedewald formula is useless in rats as in humans. Nowadays, there are direct homogeneous measurement  automatized methods  for LDL-Chol, but I do not know if  those methods are useful for rats. 

Discussion is here developed in two directions - whether is the Friedewald formula suitable for rats, and secondarily whether is the Friedewald formula suitable, nowadays, in humans.

I want to comment the second topic. I disagree with the view that direct methods of measuring LDL-C are a good or better alternative. They are not! There are several well-documented publications that describe no consensus in the results obtained with the diagnostic sets of different manufacturers. Direct methods are not mutually harmonized! LDL-C, measured by the method A is regularly different from the method B, C or D, in terms of tens of percent. Therefore, in clinical practice and in epidemiological studies, is yet still preferred Friedewald calculation. A large part of the criticism against the Friedewald has yet solved by deriving and describing of Martin's methods - article citation I wrote in the previous post.

I am extremely thankful to all the contributors, indeed all your discussion made me understand about this subject. If you have free time, please read this research article named "Do not use the Friedewald formula to calculate LDL-cholesterol in hypercholesterolaemic rats".

This article gives highlight to our discussion.

Once again, i would like to convey my sincere thanks for all the researchers.

I guess the Friedewald formula is useless in rats. However, I know that some researchers have used the formula to calculate LDL-cholesterol in short, but they were wrong, at least in my opinion. What about to use the formula in Watanabe hypercholesterolaemic rabbits?

Cut OF value for TG in young male adults Sprague Dowley rats was 42+/- 12 mg/dL X+/SD in our laboratory of Lipids and Lipoproteins-INFIBIOC.University of Buenos Aires-in this case, I prefer the UC method for separation of lipoproteins instead of Friedewald formula. 

I don't know if my answer comes too late but i just wanted to share with you a personal experience. In our laboratory, comparing the results of cholesterol contents in LDL fraction isolated by precipitating method (burstein) or FPLC, and trying to apply the Friedewald formula, I noted that this equation can't be applied in rats.

the main argument , i think, that could explain this is that the major cholesterol transporter in rats after the HDL fractions is VLDL (not LDL like in human) due to a high activity of LRP and the absence of CETP, and maybe to other parameters like, for exemple, liver cholesterol and biliary acids metabolism

this reference could help you to understand very well the major differences in lipoprotein metabolism between human and rodents

http://geronj.oxfordjournals.org/content/39/5/513.abstract