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How much fasting hours exactly necessary for accurate LDL measurement? How much difference between measured and calculated generally tolerable?

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Pathologist and Microbiologist
Practicing since : 2006
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Dear Sir,     

I am patient but this one is not health query , it is a technical query related to analysis of assay of LDL- Cholesterol.

I have to analyze lipid profile in two different labs because of situation based. However both the labs reports Total – Cholesterol and HDL similar & satisfactory , but the concern is only for LDL.

LDL can be measured by using Friedewald formula or can be estimated by real assay measurement by one of the below method.

CHE-Sterase Peroxide ( Used by my city lab)
Homogenous enzymatic colorimetric assay ( Used by Thyrocare)

In my city a distinguish lab reports LDL in both the way (calculated by using Friedewald formula ) and also ( measured by using CHE-Sterase Peroxide method.) Surprising thing is that the values calculated Vs. Measured are very similar or having  2-4 difference.
How come is it possible by any method to estimate LDL so closely.?

When Thyrocare ( a reputed lab in India) reports by above method , there is a large gape between measured and calculated values. Say 75 (measured) Vs. 90 (calculated) mg/dL.

I know that 10-12 hrs fasting is necessary for analysis. However unknowingly I went with 14-15 hrs fasting which drastically reduced by Triglycerides , resulting when LDL calculated , LDL always comes higher. I was anxious because I was on therapy.

My query questions are follows.

1.How much fasting hours exactly necessary for accurate LDL measurement ?

2.What is the effect of LDL assay on fasting hours ? I know that TG reduce drastically from 10th to 15th hrs.

3.Which readings are more reliable , Indirect (calculated) or direct (measured)?

4.If Direct ( measured) is more reliable which of the above two mentioned method is accurate for LDL measurement ?

5.How much difference between measured and calculated is generally tolerable ?

As a Pharma- R & D professional , I am so bothered because between these labs the LDL values differences sometimes are 25-30 mg/dL . Which mislead therapy duration and dose. And I can not afford long term statin therapy due to it’s hepatic toxicity.

An OPD Physician generally not bothered about the calculated / measured and analysis method used, he decides based on figures only.

So I am so eager to know answers of above queries.

Thanks & Regards.
Posted Fri, 19 Jul 2013 in General Health
Answered by Dr. Shailja Puri 1 hour later
Thanks for posting your query on Health care magic.
I will answer your queries in the order asked by you.
1.     Ideally, a fasting period of 12 hours is required before taking up any lipid test. However fasting is not required for estimation of cholesterol and HDL levels. Fasting is a must for triglyceride and LdL estimation.
2.     The fasting hours can vary from 9-12 hours for the lipid test. Fasting hours below this limit are no recommended as the value of TGs and LDL can be higher than the actual value. Fasting hours above this limit are not required as there is little change in the value of these parameters above this limit. It leads to unnecessary inconvenient fasting state in the subject.
3.     The calculated LDL is being reported by majority of labs these days. This LDL is measured from various parameters- total cholesterol, HDL and TGs. However, direct measurement of LDL is done under special condition. When the TG level is raised, LDL is estimated by direct method. Because the LDL value can be misleading by the calculation method if TGs are not in normal range.
4.     Indirect method of calculation method is commonly used for LDL estimation when TG levels are within normal limits. The direct method is used when TG level is abnormal.
5.     The difference between calculated and directly measured LDL will depend upon the TG levels.

If you have more queries, I will be glad to answer.
Dr Shailja P Wahal
Above answer was peer-reviewed by
Follow-up: How much fasting hours exactly necessary for accurate LDL measurement? How much difference between measured and calculated generally tolerable? 2 hours later
Respected Madam,     

Thanks for the technical justifications and answers. OK now onwards I will follow strict 12 Hrs fasting .

1.First Day : When I fast more than 14 -15 hrs my TG remains low as 84 mg/dL , but the measured and calculated both LDL = 100 mg/dL was reported by my city lab using CHE-Sterase Peroxide method.

2.On next day when I given sample to thyrocare lab after 9.5 hr fasting my TG remains high 166 mg/dL but the measured LDL-Direct was 76 mg/dL only , while calculated was 90 mg/dL reported by Thyrocare lab using Homogenous enzymatic colorimetric method.

Why there is a lot of difference between two lab readings , are due to method of analysis or effect of fasting hours.

I am confused with analysis ( 100 Vs 76 mg/dL) , because statin therapy depends on amount of LDL assay.

At last which one is the more accurate method for direct measurement as per your experience?

Answered by Dr. Shailja Puri 6 hours later
Hello and welcome again,
A fasting period of 9-12 hours should be followed.
A necessary pre-requisite for any test is to maintain a uniformity.
TG levels vary with fasting hours.
These tests which are used to moniter the drug therapy should have uniform pre-analytic variables.
Else the readings may be mis-leading.
So, next time get the TG levels done after 9-12 hours of fasting.
Regarding a variation in LDL levels, it should be noted that the results of the tests depend on the precision of the instrument.
It is better to get the tests in one reputed laboratory maintaining uniform pre-analytical variables.
If the results are are abnormal at any instance, get it repeated.
If the value for the test remains abnormal, drug has to be titrated accordingly.
A variation of 14 mg/dl (76 and 90mg/dl) is too wide to be acceptable.
It is advicable to get both the tests repeated after 9-12 hours of fasting, from same laboratory once again and then observe the results.
If the results remain abnormal you will have to consult your physician for drug titration and/ or life style modifications.
If you have more queries, I will be glad to answer.
Dr Shailja P Wahal

Above answer was peer-reviewed by
Follow-up: How much fasting hours exactly necessary for accurate LDL measurement? How much difference between measured and calculated generally tolerable? 37 hours later

Thanks for the answer.

I will follow fasting exactly 12 hrs. fasting.

Test other than Cholesterol , thyrocare and my city lab is almost matching but in case of LDL ( direct measuremrnt) it differs. 76 Vs 100 mg/dl

However the values of calculated LDL are seems to be near that is
90 Vs. 100 mg/dl of both the lab respectively.

I think I should consider finally calculated values because which are near by that is 90 to 100 mg/dl LDL ( calculated) respectively.

regards and thanks again.

Answered by Dr. Shailja Puri 9 hours later
Hello and welcome again,

I guess you are facing this discrepancy for the first time.
Don't panic.
Continue with your medicine.
Next time get the test done at suitable interval and observe the values.
Direct measurement is a fairly accurate method of estimating LDL values.
However, if other parameters, i.e. total cholesterol and TGs are within range, calculated values are also quite accurate and reliable.

Keep these general points in mind for the future.
Wish you good health.
Thanks and take care
Dr Shailja P Wahal
Above answer was peer-reviewed by
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