Section F8(vii) from the 2017 CICM Primary Syllabus expects the exam candidates to be able to "describe physiology and consequences of abnormal haemoglobin", and this chapter is related to it only through major deformation of what one would normally accept as definitions of "related" and "is". In short, the haemoglobin measurement from the ABG sample can be processed mathematically to produce a haematocrit value. The ABG machine reports this variable as Hctc - the little "c" representing the fact that this variable has not been measured directly, but was calculated using an equation and certain assumptions (the gold standard of haematocrit measurement being of course the old-school centrifuge).
The Haematocrit Equation
The local unit uses the following algorithm to calculate the haematocrit value:
Hctc = 0.0485 × ctHb + 8.3×10-3
In the operations manual, the reference given to support this algorithm is an article by G.Kokholm from 1990. I have no access to it, and therefore I cannot elaborate on how exactly those fudge factors were determined. Luckily, the ancients have provided me with a dusty tome from 1965, the 8th volume of the "Advances in Clinical Chemistry" book series (which is now into Volume 66). Within this grimoire, one can find a review article by E.J. van Kampen and W.G. Zijlstra,"Determination of haemoglobin and its derivatives", which turned out to be a complete goldmine of information.
Hematocrit determines the fraction of the blood that is red blood cells, and red cells are about 95% haemoglobin by weight (dry mass). Thus, one can say the hematocrit is directly proportional to the total haemoglobin value. A rough estimate is that the hematocrit (in percent) is roughly three times the hemoglobin concentration in g/dl.
This relationship is valid provided certain assumptions are true:
- The haemoglobin value has been calculated correctly (i.e. there are no interfering substances)
- The red cells contain an expected amount of haemoglobin (i.e. the MCHC is normal)
- There is no "free haemoglobin" (i.e. there is no intravascular haemolysis) to confuse the analyser.
Thus, a person with raging haemolysis will have enough free haemoglobin measured in the ABG machine to generate a normal-looking calculated haematocrit, even though their measured haematocrit may be desperately low.
Similarly, a patient with a hypochromic anaemia and a normal measured haematocrit will end up having an inaccurate low calculated haematocrit, because the haemoglobin value is lower.
However, in the latter scenario, the trend in haematocrit will still be valid as an answer to the usual question we ask of the haematocrit, which is "is my patient bleeding to death?"
Empirical evidence of the (in)accuracy of ABG haematocrit measurements
In 2010, Bosshart et al from the University Hospital of Zurich published on this topic: "In a total of 50 critically ill patients hematocrit was analyzed using (1) blood gas analyzer (ABLflex 800) and (2) the central laboratory method (ADVIA® 2120) and compared."
The calculated difference in haematocrit percentages was +/- 1.4%. The authors caution that if one were using the haematocrit as a transfusion trigger, one's transfusion practice might be substantially influenced by one's choice of measurement method. This is supported by an earlier study (using a different ABG analyser in the context of cardiopulmonary bypass) which demonstrated that at low haematocrits (eg. below 30%) the ABG machine indeed provided a down-estimated result, resulting in an increased number of transfusions.
Of course, the pragmatic intensivist will belch a filthy profanity in response to all this, and dismiss these laboratorian concerns with the statement that the practice of using haematocrit (or haemoglobin for that matter) as a transfusion trigger is a ridiculous practice, and that one's decision to transfuse the patient is a raw gut-based clinical bedside get-your-hands-on-the-patient-and-assess-them-like-a-fucking-doctor sort of decision.