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Chapter question: Every laboratory test at the bedside was validated by changing one thing and watching the number move. An ECMO patient changes everything at once — haemoglobin, platelets, fibrinogen, factor levels, haematocrit, temperature, heparin and free haemoglobin, all in the same twelve hours. What survives that, what does not, and how should the panel be read?
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Evidence search date: 22 September 2026.
Primary sources: ELSO Red Book 6th edition — the adult cardiac chapter (Table 28-2, the laboratory frequency schedule adapted from McMichael; Table 28-3, transfusion thresholds; the consumptive-coagulopathy panel; factor XIII and acquired von Willebrand syndrome) and the neonatal chapter (Table 9-1, the haematologic laboratory schedule; what the activated clotting time is affected by). ISCCM Manual of RRT and ECMO in ICU — the coagulation monitoring table with its targets, advantages and disadvantages, and the unit's own paediatric monitoring protocol. ECMO in the Adult Patient (Core Critical Care, 2017) — the haematology chapter: assay interference by free haemoglobin, the blood film, thrombocytopenia, platelet increments, red cell antibodies and the factor XI case. ECMO: A Practical Guide to Management (Taha, Caridi-Scheible, Leiendecker) — the haemolysis marker set, the two mechanisms of free-haemoglobin organ injury, and the coagulation test comparison table.
External evidence: an in vitro and in vivo study of four haemostasis assays against isolated and simultaneous changes in heparin and coagulation factors; an adult venoarterial cohort of low-level haemolysis and stroke; a paediatric cohort of haemolysis and acute kidney injury; a single-centre venovenous haemolysis analysis; a 215-patient study of free haemoglobin before and after cannulation; a pump comparison; a carboxyhaemoglobin study; two studies and two conference abstracts on acquired von Willebrand syndrome and platelet receptor shedding; a point-of-care free-haemoglobin method comparison; and two haematology reviews.
Every external reference was read as an indexed abstract or full-text passage. None was retrieved as a complete article. Digital object identifiers are reproduced from the search record and have not been independently resolved. Every database row seeded from this chapter is marked Verified = No.
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This is the fifth chapter of Part V and the most tightly hemmed in, because four other chapters own pieces of the laboratory. The line this chapter holds is between the test and the thing the test is about. Chapter 37 owns haemolysis as a complication to be prevented and treated; this chapter owns the measurement of haemolysis, and what that measurement does to every other measurement. Chapter 42 owns anticoagulation monitoring as a therapeutic decision; this chapter owns the assays as instruments, and what they are actually correlated with.
| This chapter owns | Owned elsewhere — cite, do not re-argue |
|---|---|
| What the circuit does to a sample before the laboratory sees it — sampling site, dilution, heparin contamination, timing | Blood gas sampling sites and their four questions — Chapter 27 |
| The haemolysis indices as measurements: plasma free haemoglobin, lactate dehydrogenase, haptoglobin, bilirubin, phosphate, the blood film, carboxyhaemoglobin — and the specific confounder attached to each | Haemolysis as a complication — its causes, its prevention, the circuit change — Chapter 37; the carboxyhaemoglobin line on the co-oximeter — Chapter 27 |
| Free haemoglobin as an assay interferent — the tests it breaks and the direction it breaks them in | The renal consequences of free haemoglobin — Chapter 55; the neurological consequences — Chapter 28 |
| The full blood count on a circuit, and the blood film | Transfusion thresholds and strategy — Chapter 46 |
| Acquired von Willebrand syndrome and platelet receptor shedding as laboratory diagnoses — which test makes the diagnosis and which test cannot | The coagulation physiology that produces them — Chapter 39; bleeding as a complication — Chapter 36 |
| What each coagulation assay is actually correlated with, measured directly | Choosing and titrating an anticoagulant against those assays — Chapter 42, Appendix 6 |
| Assembling a laboratory schedule, and the cost of the panel itself | Integrating the whole dataset and the defect classes of interpretation — Chapter 30 |
This is the organising idea, and unusually for this book it has been measured directly rather than argued.
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Evidence · The study that should change how the coagulation panel is read
What was done. Four assays — prothrombin time, partial thromboplastin time, activated clotting time and anti-factor Xa heparin activity — were tested against isolated changes in heparin and coagulation factor levels, and then against simultaneous changes, using both in vitro samples and in vivo samples from patients on extracorporeal life support.
What was found with one variable changing at a time.
What was found when multiple parameters changed, in vitro and in vivo.
The authors' conclusion, reproduced because it is unusually blunt for a laboratory paper: the activated clotting time is "too insensitive to heparin, sensitive to too many other factors, and too imprecise to be useful for monitoring hemostasis during ECLS."
Certainty: moderate to high for the correlation structure, which is a measurement rather than an inference and was reproduced in vitro and in vivo. Low for the practice recommendation, which no outcome study supports.
Source: Saifee NH, Brogan TV, McMullan DM, Yalon L, Matthews DC, Burke CR, Chandler WL. ASAIO J 2020;66(2):230–237. DOI 10.1097/MAT.0000000000000993.
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Physiology · Why a good assay becomes a bad measurement
The finding above is not a statement about laboratory quality. Every one of those assays performed exactly as designed. What failed was the inference from the assay to the patient, and the reason is worth stating in general form because it applies to everything in this chapter.
A clotting assay is a race. A reagent starts a reaction and the instrument reports when a clot forms. That time depends on everything in the tube at once — the concentration of each factor, fibrinogen, platelets, the haematocrit that determines how much plasma there is per millilitre of sample, the temperature, and any drug that inhibits any step. A validation study varies one of those and shows a tight correlation. That correlation is real, and it is conditional.
An ECMO patient violates the condition continuously. In a single twelve-hour period the same patient can have a falling haematocrit, a falling platelet count, a falling fibrinogen, consumed antithrombin, a changing heparin infusion, a rising free haemoglobin, a temperature set by a heat exchanger and two units of red cells and a unit of plasma delivered. Every one of those moves the clotting time, and the assay reports a single number that cannot distinguish them.
The consequence is a rule, not a caveat: a test whose result depends on several variables is uninterpretable in a patient in whom several variables are moving, no matter how good the assay is. The tests that survive are the ones that are specific to a single quantity — anti-Xa for heparin, prothrombin time for factor levels — and the way to survive with the others is to measure more than one and read them against each other.
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Clinical pearl · The one-line version, for the ward round
"This number moved. Which of the six things that move it, moved?"
It is worth asking out loud, because the reflex answer on ECMO is always the heparin, and the heparin is frequently not the answer. A rising activated clotting time in a patient whose platelets have halved and whose fibrinogen is 1.1 g/L is describing the platelets and the fibrinogen at least as much as the infusion, and turning the heparin down will not correct it.
This chapter's sections are, in effect, six answers to that question for six panels.
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Pitfall · Treating the reference interval as if it applied
Every reference interval on the report was derived from a population of healthy volunteers who were not anaemic, not transfused, not anticoagulated, not febrile, not in multi-organ failure, and not connected to a pump. The interval is a statement about them, not a statement about this patient.
That is true in any intensive care unit. What makes it sharper on ECMO is that the deviations are not random — they are systematic, they are in a known direction, and they are produced by the treatment. A haematocrit of 0.28 is not noise; it is the expected consequence of circuit priming and haemodilution. A raised lactate dehydrogenase is not an incidental finding; it is what a pump does.
So the useful comparison on ECMO is almost never the reference interval. It is this patient's value yesterday — which is the same rule Chapter 25 reached for haemodynamics, Chapter 26 for ejection fraction and Chapter 28 for cerebral monitors. Part V has now arrived at it from five directions.
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Chapter 27 established that a blood gas is uninterpretable without its sampling site. The same is true of the laboratory panel, for different reasons, and the errors are more easily made because a full blood count does not obviously belong to a site the way a gas does.
| Pre-analytical problem | What it does to the result | What to do |
|---|---|---|
| Heparin contamination from a circuit sampling port | Prolongs every heparin-sensitive clotting time, sometimes dramatically. A coagulation sample drawn through a heparinised line is not a coagulation sample | Draw coagulation studies from a peripheral or dedicated arterial line where possible; if the circuit must be used, discard an adequate volume first and record that the sample was circuit-drawn |
| Haemodilution from the circuit prime and from fluid | Lowers haematocrit, platelet count, fibrinogen and every factor level together, in proportion. A proportional fall across the whole panel is dilution until proven otherwise | Read the panel as a pattern. An isolated fall in one component is a finding; a fall in all of them at once is arithmetic |
| Mechanical haemolysis during the draw itself | Raises free haemoglobin, potassium and lactate dehydrogenase in the tube, not in the patient. Indistinguishable on the report from the haemolysis this chapter is trying to detect | This is the single most important artefact in §29.3. A first abnormal free haemoglobin should be repeated, from a different site, before it is believed |
| Iatrogenic blood loss from the panel itself | The schedules in §29.12 draw blood many times a day. Over a week that is a measurable contribution to the anaemia the panel is being used to monitor | Use paediatric tubes where the assay allows; audit the daily phlebotomy volume as a number, because no one does |
| Timing relative to a heparin change | An anti-Xa drawn before a dose change has equilibrated is describing the previous dose | The Red Book schedule states it: anti-Xa 4 hours after each heparin change |
| Timing relative to transfusion | A haemoglobin drawn during a transfusion is a number in transit. Haptoglobin falls after red cell transfusion independently of haemolysis (§29.3) | Record what was transfused in the preceding hours alongside the panel |
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Danger · A single abnormal free haemoglobin has an artefactual explanation that is commoner than the real one
Every part of this chapter that follows depends on the free haemoglobin being real. A difficult draw through a small needle, excessive suction, or a long transit produces exactly the same result as pump-induced haemolysis, and in most laboratories the two are not distinguishable on the report.
Before a rising free haemoglobin triggers a circuit change — which is what Chapter 37 will have it trigger — repeat it, from a different site, and look at the concordant markers. Real intravascular haemolysis on a circuit moves a set of numbers together: free haemoglobin up, lactate dehydrogenase up, unconjugated bilirubin up, phosphate up, haptoglobin down, and frequently the urine or the dialysis effluent visibly discoloured. A high free haemoglobin with a normal lactate dehydrogenase, a normal haptoglobin and clear urine is a sampling artefact until proven otherwise.
Source for the concordant marker set: ECMO: A Practical Guide to Management, the venovenous physiology chapter, which lists plasma free haemoglobin, lactate dehydrogenase, unconjugated bilirubin and phosphate rising with haptoglobin falling, and recommends trending them rather than reading single values.
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Haemolysis is the laboratory signature of extracorporeal support, and it is measured with five or six tests that are quoted as a set. Read individually, every one of them is confounded by something an ECMO patient definitionally has.