Fast­ing ver­sus non-fast­ing tests

Daily Freeman (Kingston, NY) - - YOUR DAILY BREAK - An­thony Komaroff AskDr.K

I heard that fast­ing will no longer be re­quired be­fore a choles­terol test. Will the re­sults still be as ac­cu­rate?

To an­swer your ques­tion, I need to first de­scribe what a “choles­terol test” is. There are three types of choles­terol that typ­i­cally are mea­sured: LDL (“bad”) choles­terol, HDL (“good”) choles­terol and to­tal choles­terol (ba­si­cally, the sum of LDL and HDL). There is a fourth type of fat mea­sured at the same time: triglyc­erides. Most doc­tors or­der all four tests as part of what’s called a “lipid (fat) panel.”

LDL, HDL and to­tal choles­terol lev­els are not much af­fected by hav­ing re­cently eaten, but triglyc­erides are. If mea­sur­ing the triglyc­erides is not that im­por­tant, then fast­ing be­comes less im­por­tant. Fast­ing is a has­sle, for both pa­tients and doc­tors. It may be a has­sle for you to come for a blood test first thing in the morn­ing, and then to have break­fast, and then to be­gin your day. And if your sched­ule doesn’t al­low you to have the test un­til later in the day, con­tin­u­ing to fast be­comes quite un­pleas­ant. In­deed, if you have di­a­betes, long fasts can be dan­ger­ous.

In­ter­na­tional guide­lines re­cently pub­lished in the Euro­pean Heart Jour­nal be­came the lat­est of­fi­cial rec­om­men­da­tion against rou­tine fast­ing for choles­terol tests. I spoke to my col­league, Dr. Naomi Fisher, an en­docri­nol­o­gist at Har­vard-af­fil­i­ated Brigham and Women’s Hos­pi­tal. She noted that these guide­lines de­fend what many health care sys­tems and doc­tors have been prac­tic­ing for sev­eral years al­ready. That’s be­cause many stud­ies in re­cent years have found that eat­ing has only slight, clin­i­cally in­signif­i­cant ef­fects on to­tal, HDL, and LDL choles­terol.

Per­haps more im­por­tant, large-scale analy­ses have shown that non-fast­ing lipid mea­sure­ments don’t weaken the abil­ity of lipid lev­els to pre­dict harm­ful events like heart at­tack and stroke. In fact, post-meal mea­sures are thought to strengthen their pre­dic­tive abil­ity. This ob­ser­va­tion may stem from the fact that most peo­ple eat sev­eral meals plus snacks dur­ing the day. That means we spend most of our time in a “fed” state, not a fast­ing state. So lipid lev­els after eat­ing may best re­flect our nor­mal phys­i­ol­ogy.

Hav­ing said this, I still ask some pa­tients to come in the morn­ing for fast­ing blood tests. First, high lev­els of triglyc­erides do in­crease the risk of heart disease. Very high lev­els can cause other health prob­lems, in­clud­ing in­flam­ma­tion of the pan­creas. So if a per­son’s triglyc­erides tend to be high, I like to mon­i­tor them in the fast­ing state so that the re­sults can fairly be com­pared to pre­vi­ous fast­ing re­sults. Also, a fast­ing blood sugar (done on the same blood sam­ple) is valu­able in mon­i­tor­ing peo­ple at risk for get­ting di­a­betes. So there can be two rea­sons, in cer­tain pa­tients, to ob­tain a fast­ing blood sam­ple.

So ask your doc­tor if you re­ally need to skip break­fast be­fore your next blood draw. There may be a good rea­son, like the ones I’ve just given. But there may not be. Tra­di­tions die hard. But sci­ence even­tu­ally pre­serves only the tra­di­tions with proven value.

Dr. Komaroff is a physi­cian and pro­fes­sor at Har­vard Med­i­cal School.)

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