Hermione is a 15-day-old baby girl brought in for prolonged jaundice. She is breastfed and has no other risk factors. Her examination is normal other than being a bit on the yellow side. You ask the nurse to perform a blood gas to check her bilirubin, which is below 200. You notice the lactate on the gas is 4, but the nurse reports it was a “squeezed sample”, which she suggests could explain the result.
The pre-analytic phase
It is not uncommon to get unexpected abnormal results from tests we perform (we do perform a lot of tests, after all), and one of the most common reasons (aside from the test actually being abnormal) is an issue in the pre-analytic phase, referring to everything that happens before your sample actually gets tested. Many things can alter various test results in the pre-analytic phase, but today, we are focussing on blood lactate. This is as a result of me confidently commenting on a Twitter post that a “squeezed sample” could result in a spuriously elevated lactate result and being challenged to back it up. This resulted in a very emotional journey full of ups and downs. Let me show you where I’ve been…
When we talk about a “squeezed sample”, what do we actually mean? Usually, we are referring to the process of squeezing a child’s limb too hard when acquiring a blood sample (either from a heel/finger stick or squeezing whilst holding for phlebotomy), which commonly results in haemolysis, which is the breakdown of blood cell walls, causing a spillage of all their inner contents.
This is a problem because there is a huge difference in concentration between some important biochemical constituents on the insides of cells and in serum. Suppose we suddenly break open all the cells and spill their guts into the serum. In that case, we can drastically alter the concentrations of these things in our sample, giving you a spuriously abnormal result in 3 ways:
- Significantly increase measurements of solute with high intracellular concentrations, including potassium and lactate dehydrogenase (LDH) in particular, but also AST, and to a lesser extent, creatinine kinase, magnesium, phosphate and calcium.
- The change in solute load can also decrease concentrations of other things in serum, including glucose, sodium, albumin, ALP, bilirubin, chloride and GGT.
- Finally, intracellular components (including haemoglobin) can ruin analysis by reacting with reagents in your specimen bottles or disrupting the spectroscopy/photometry detection methods. This can affect the creatinine kinase, bilirubin, iron, lipase and GGT results.
It’s worth noting that most test results are not affected by haemolysis unless it is extreme, so unless you have put your sample through a blender, the lab can give you that CRP you wanted.
What about lactate?
There is surprisingly little evidence on the effect of haemolysis on lactate, but one of the reasons is that haemolysis does not appear to affect lactate results. This was demonstrated in a study demonstrating no correlation between potassium and lactate on blood gases and a study in pigs, which assessed the effects of haemolysis on various metabolites, including lactate, demonstrating no effect.
But the story doesn’t end there…
Squeezing for prolonged periods causes venous stasis (which is obviously the whole point), which in itself can significantly alter local haemodynamics. This has been shown to increase lactate levels when prolonged. In adults, there appears to be little effect in up to 5 minutes of tourniquet application; however, we definitely squeeze children tighter than a tourniquet…
Squeezing above the mean arterial pressure can also impair local tissue perfusion, which can significantly increase local blood lactate results in the setting of ischaemia as hypoxic tissues undergo anaerobic respiration.
Distress and exertion
Most importantly, we should remember the reason why we are (or as a result of) squeezing so hard is that the child is distressed and sometimes vigorously resisting the procedure. Exertion can have dramatic results on blood lactate levels, increasing lactates into the high teens in adults after exercise, and even normal exertion has been demonstrated to significantly increase lactate in children compared to 40 minutes of bed rest.
As an additional note (unrelated to squeezing), one of the most important pre-analytic effects on lactate results is time. When blood is drawn, red cells continue with glycolysis. This means they continue to produce lactate even when out of the body, so if your sample is left to sit for too long, the lactate could continue to rise until it is finally analysed. This issue can be ameliorated by sending a sample in an oxalate tube or by sending the sample on ice, both of which inhibit red cell glycolysis.
Squeezing of a limb when acquiring a blood sample can spuriously increase lactate by:
– Venous stasis
– Impaired local perfusion
Distress/exertion of the child
Haemolysis does not affect lactate
Prolonged sample storage will spuriously increase lactate unless sent in an oxalate tube or on ice