Ischaemic Lactate Test
Suspected metabolic muscle disease.
This protocol is from Professor Land at Queen's Square.
Warn biochemistry 24 hrs prior to test that assays for pyruvate, ammonia and lactate will be required.
Tubes for pyruvate: Tubes prepared in the lab by the addition of 2mls perhexilene and refrigerated overnight. Add 1ml of blood to each tube accurately. Specimens for pyruvate must be handled carefully and placed on ice and taken to the lab immediately.
Tubes for lactate: Grey top fluoride oxalate bottles (samples stored on ice).
Tubes for ammonia: 9 paediatric lithium-heparin tubes (samples stored on ice). The 9th tube is a control, to measure the background ammonia levels in the samples.
- Fast from midnight.
- The patient must spend the day relaxing, not doing any exercise.
- 2 people needed to assist with sampling.
- Insert i.v. cannula into large forearm vein with a three-way tap.
- All specimens should be free flowing blood.
- Take baseline bloods (-2 min) for lactate, pyruvate, ammonia, CK, phosphate and uric acid. At each time point discard 3mls of blood from the cannula, take 1ml for pyruvate in a 2ml syringe so that the volume is accurate, and 6mls in a 10ml syringe for the rest. Flush the cannula with normal saline and put the bottles on ice immediately.
- Place sphygmomanometer on the cannulated arm and inflate the cuff above systolic pressure. The patient exercises the arm rhythmically by squeezing some rolled up paper towels or a ball. The hand must be fully extended between squeezes. Exercise the hand for 2 minutes.
- Release the cuff, this is time = 0.
- At time 0, 1, 2, 4, 6, 8 and 10 min take blood for lactate, pyruvate and ammonia as above.
- Normally the lactate rises by 3-5 x baseline.
- The ammonia rises from 40 µmol/l to about 100 µmol/l.
- The normal lactate:pyruvate ratio is 10-20 which rises to 30-40 on exercise.
The lactate test is positive when the patient exercises and they can't open their hand fully. The lactate level remains unchanged, as glycogenolysis is defective. The ammonia level rises dramatically to 300-400 µmol/l. The lactate to pyruvate ratio is 10-30 and does not change on exercise.