LD Blood Work: What an LDH Test Measures (Not Lactate)
LD blood work measures the enzyme lactate dehydrogenase, not blood lactate. How to read the lab range, what affects it, and what to log before the draw.
LD blood work is a venous blood test for lactate dehydrogenase (LD or LDH), an enzyme found in many different tissues of the body (URMC Health Encyclopedia). When those cells are damaged, LD can show up in the blood at higher levels, so the result works as a broad signal that a clinician reads together with other findings. It is reported in units per liter. It is not the fingertip blood lactate value endurance athletes measure during a step test. Different molecule, different unit, different question.
LD and blood lactate are two tests that share a word
The names overlap because LD is the enzyme that converts between pyruvate and lactate. That is where the similarity ends. One test asks how much of an enzyme is circulating; the other asks how much of a metabolite has accumulated at a given workload.
| LD blood work | Blood lactate (step test) | |
|---|---|---|
| What is measured | Activity of the enzyme lactate dehydrogenase | Concentration of lactate |
| Unit on the report | U/L or IU/L | mmol/L |
| Sample | Blood drawn from a vein | A capillary drop, usually from a fingertip or earlobe |
| Who usually orders it | A clinician, as part of lab work | An athlete, coach or test service |
| What the number is used for | One input to a medical evaluation | Locating LT1, LT2 and training zones |
| Repeated within one session? | Typically one draw | One reading per stage, often 8–12 strips |
The URMC page describes the sample as blood drawn with a needle from a vein in the arm or hand (URMC Health Encyclopedia). The UCSF Benioff Children's Hospital test description calls LDH a protein that helps produce energy in the body (UCSF Benioff Children's).
The mix-up that breaks a lactate analysis
The typical failure is a unit error, not a medical one. Someone finds "LD 190" on an annual lab report and enters it into a lactate calculator as 190 mmol/L. In a typical fictional step test, readings run from about 1.1 to 8.4 mmol/L. An LD value of 190 is more than 22 times the highest of those readings. No curve fit survives that.
The reverse happens too: an athlete asks a doctor's office for "a lactate test" to get training zones and receives an LD result with a reference range. That number cannot be turned into zones. Zones need a series of lactate readings matched to power or pace.
How to read the number on your report
The reference range belongs to the lab, not to the internet
The UCSF page gives a normal value range of 125 to 220 international units per liter and states that ranges may vary slightly among laboratories, because some labs use different measurements or test different samples (UCSF Benioff Children's). The URMC page lists 105 to 233 units per liter for ages 16 and older (URMC Health Encyclopedia). Two published adult references, two different upper limits: a value that sits inside one lab's range can be flagged by another.
Decision rule: compare your value only against the range printed on the same report. A range copied from a web page is a reference for orientation, not for a verdict.
Where your value sits: a position calculation
A single number is easier to discuss when you know where it falls inside its own range. Use this:
Position (%) = (your value − lower limit) ÷ (upper limit − lower limit) × 100
Example with a fictional value, A: a result of 225 U/L against the URMC range for ages 16 and older (105–233) gives (225 − 105) ÷ 128 = 94 %. Inside the range, near the top.
Example B: the same 225 U/L against the UCSF range (125–220) gives (225 − 125) ÷ 95 = 105 %. Now it sits above the range.
Same number, two published ranges, two different flags. That is why the printed range matters more than any number you memorize. Values above 100 % or below 0 % are outside the lab's range, and what that means is a conversation for whoever ordered the test.
What can raise LD besides the reason the test was ordered
Because the enzyme sits in so many tissues, the test is not specific to one organ. The UCSF description lists a wide spread of conditions associated with higher-than-normal levels, including heart attack, hemolytic anemia, liver disease, muscle injury, pancreatitis and stroke. (UCSF Benioff Children's). URMC adds that a higher total LDH does not show which tissue or organ is affected, and that moderate to high levels are sometimes found in healthy people during routine lab work (URMC Health Encyclopedia). Kaiser Permanente's patient page says LDH levels help find the cause of lung disease, lymphoma, anemia and liver disease, and notes that a high result can also happen because of exercise or a technical problem with the test (Kaiser Permanente).
For anyone who trains hard, the exercise line is the practical one. It does not tell you what your number means. It tells you which context the person reading it needs.
The pre-draw log for athletes
This is a record, not a protocol. Write it down before the appointment and hand it over with your question.
- Hours since your last hard session (intervals, race, long run).
- Session type: steep downhill running and heavy strength work are muscle-loading in a way easy riding is not.
- Any step test or race in the past seven days, with the date.
- Current muscle soreness, yes or no, and where.
- Medicines and supplements taken in the past week, including over-the-counter pain relievers.
- Whether the draw was fasting, and the time of day.
Ask the clinician whether any of these should change when the blood is drawn. Do not decide that yourself, and do not reschedule a test someone ordered for a medical reason because of a training plan.
Timing in December and January
The turn of the year is when two calendars collide for many endurance athletes: routine checkups and lab work, and the indoor base block where a first step test of the season often lands. If both fall in the same week, put both dates on the log above. A fictional rider who does a ramp test on a Monday and has blood drawn on Tuesday morning gives the reader of that result roughly 18 hours of context they would otherwise not have.
The same logic runs the other way for your lactate data. If you are unwell, recovering from a hard holiday race, or have just changed medication, note it in the test record. A step test done in that state is still data, but it should not be the baseline you compare the rest of the season against.
Questions people ask about LD results
Does a high LD result mean cancer?
Not by itself. LD rises with many different conditions. Most entries on the UCSF list of causes of higher levels are not cancers, for example hemolytic anemia, liver disease and muscle injury, although leukemia, lymphoma and other cancers are on the list too (UCSF Benioff Children's). Kaiser Permanente names lymphoma among the conditions the test helps evaluate (Kaiser Permanente). Which explanation applies to a specific result is a question only the ordering clinician can answer, with the rest of your history and tests in view.
What counts as a high LD level?
A value above the upper limit printed on your own lab report. The UCSF example range is 125 to 220 IU/L, with the explicit note that ranges vary among laboratories (UCSF Benioff Children's); URMC lists 105 to 233 units/L for ages 16 and older (URMC Health Encyclopedia). Use the position calculation above to see how far outside the range a value sits.
Is a low LD result a concern?
The patient sources cited here focus on elevated values. URMC only notes that medicines containing ascorbic acid (vitamin C) can lower LDH levels. If your report flags a low value, ask the person who ordered the test what it means in your case rather than searching for a general answer.
Can I set training zones from LD blood work?
No. Training zones come from a series of blood lactate readings taken at increasing workloads, fitted to a curve to locate LT1 and LT2. If you want that measured by someone else rather than at home, compare providers using mobile lactate testing services: how they work and what to compare, then ask any service which test they actually run before you book. If the answer is "LDH", it is the wrong test for zones.