Somebody asked me last week if dehydration could throw off their labs.
Yes. So can leg day.
Your blood work measures one morning. Not you. Here is everything that can wreck the picture before the needle even goes in.
Less water in your blood means everything in it looks more concentrated.
Climbs falsely: hemoglobin, hematocrit, albumin, total protein, BUN, sodium.
The tell: they all go up together. One marker high is a finding. Six going up at once is usually a hydration story.
Do not chug a liter in the parking lot either. That will dilute your labs and create the opposite problem.
AST and ALT get called liver enzymes. They also live in muscle.
Lift heavy, and they spill into your blood, and your report looks like liver disease.
Healthy untrained men did one hour of weightlifting in a 2008 study. AST, ALT, LDH, and CK stayed elevated for at least seven days. Their GGT and alkaline phosphatase never budged, which is the clue the liver was never involved.
I have watched people get sent for liver imaging over leg day.
Skip hard training 24 to 48 hours before. Longer if you did something new.
Skip it. Details below.
Past roughly 14 to 16 hours your body starts burning fat for fuel. Free fatty acids rise, ketones rise, and bilirubin can climb high enough to flag on its own.
More fasting is not more accurate.
Adrenaline sweeps white blood cells off your vessel walls and into circulation within minutes.
Climbs falsely: WBC, neutrophils, glucose, cortisol.
If you are scared of needles, your labs know it.
Moves your glucose, your insulin sensitivity, and your cortisol. One night.
Recent illness rearranges a lot:
WBC goes up, and your neutrophils and lymphocytes shift around, which throws off your NLR
hs-CRP goes up. It starts climbing within hours and peaks around a day and a half
Sed rate (ESR) goes up and is slow to come down. It can stay elevated for weeks after you feel fine
LDH goes up, because it spills out of any damaged tissue anywhere in the body
Ferritin goes up, which can hide a real iron deficiency underneath
Albumin and cholesterol go down
Thyroid can look hypothyroid when your thyroid is fine
This is the section I want you to read twice, because these are the exact markers people run when they are worried about something serious. Draw them during a recovery and you get a picture of the illness instead of a picture of you.
Wait a few weeks after any illness. Longer before checking thyroid or inflammation. And when you look at ferritin, look at your hs-CRP sitting next to it.
The hair, skin, and nails supplement. Also hiding in a lot of multivitamins.
It interferes with the chemistry of a whole category of tests, including thyroid, hormones, ferritin, and troponin, and can push a result falsely high or falsely low.
The FDA has issued safety warnings on this twice. The second one followed a patient death after a falsely low troponin.
Stop it several days before any draw.
Iron taken that morning spikes your serum iron and makes your iron status look better than it is
Creatine converts to creatinine, which raises creatinine and drops your eGFR with nothing wrong with your kidneys
High dose vitamin C interferes with several chemistry tests
Taking it the morning of your draw spikes free T4 for hours.
Hold the dose until after. I have seen doses cut based on a number that was measuring the pill somebody swallowed 90 minutes earlier.
Tourniquet on longer than a minute concentrates the sample and can rupture red cells
Pumping your fist dumps potassium out of your forearm muscle
A shaken or delayed sample leaks potassium and LDH out of the cells
You are allowed to say “could you loosen that?” out loud.
The official guidance loosened in 2016, when the European Atherosclerosis Society said nonfasting lipid panels are fine for routine screening. The American College of Cardiology agreed.
I still say fast.
Triglycerides move a lot after you eat, and your calculated LDL is built off your triglyceride number, so one meal shifts two numbers.
And I am watching your labs across years. A fasting panel from March against a nonfasting panel from October is not a comparison. I want your trend, and a trend only means something when the conditions match.
12 hours. Water only. No coffee, no gum, no nicotine.
Nobody tells you this part.
Published error rates for the total testing process run somewhere between 0.012 and 0.6 percent of all results, and some surveys put the range as high as 3 percent. Small, and not zero.
What surprised me when I started reading this literature is where the errors happen.
46 to 68 percent of lab errors happen before your sample ever reaches the analyzer. Collection, labeling, handling, transport
Only 7 to 13 percent happen during the actual analysis. The machines are the reliable part
Mislabeled specimens run 0.01 to 2 percent depending on the setting
Hemolysis, meaning ruptured red cells in the tube, has been reported as high as 22 percent in some settings, and it falsely raises potassium, LDH, and AST
One hematology lab reviewed 231,008 samples and rejected 5.15 percent of them for pre-collection problems. Transport delays, wrong tubes, hemolysis, unlabeled samples. Those were the ones caught. The concerning ones are the samples that pass inspection and get resulted anyway.
Add that to the statistics of reference ranges and you get the whole picture.
A reference range captures the middle 95 percent of a population. By design, 1 in 20 healthy people falls outside it.
Run 20 markers on a perfectly healthy person and there is a 64 percent chance at least one comes back flagged. That math was published in the Journal of the American Board of Family Medicine in 2025.
I have seen many mistakes in my 20 years on medicine.
Before you panic, before you start a medication, before you accept a diagnosis:
Repeat the test. Hydrated, rested, fasting, no workout, no biotin, morning, same lab.
Real findings stay. Noise disappears.
One weird number in someone who feels fine is a question. The same number twice, or one number plus symptoms, is an answer.
Week before
Stop biotin and high dose vitamin C
No heavy or unfamiliar training for 24 to 48 hours
Wait a few weeks past any illness
Day before
Hydrate normally
No alcohol
Nothing unusual for dinner
Morning of
12 hours fasting, water only
Hold iron and thyroid medication until after
Same time of day, same lab, every time
Sit and breathe before they stick you
After
Compare to your own last results, not just the reference range
Repeat anything dramatic before you act on it
Most of what looks alarming on a lab report is a story about the day before the draw, and somebody should have told you that the first time they handed you a printout.
If you are looking at a result right now and wondering whether it is real, come ask.
I built Optimal Not Normal for this. Over 100 members are in there learning to read their own labs, asking questions about their results, and getting better at every conversation they have with their provider. This is where the education happens, and where I answer things I cannot fit into a post.
Join Optimal Not Normal here and learn to become the CEO of your own health.
The most radical act in a sick society is to heal yourself — and then gently help others heal too.
— Dagmara
#BeTheCEOofYourHealth
This post is for educational purposes only and does not constitute medical advice. Please work with your own healthcare provider for personalized care, and never stop or change a prescribed medication without talking to the provider who prescribed it.
No posts
Comments
Nothing yet. Say the first thing.
Sign in to join the conversation.