Laboratory assessment of liver function

This chapter tries to address Section N2(i) from the 2017 CICM Primary Syllabus, which asks the exam candidates to “describe the interpretation of laboratory assessment of liver function (Albumin, Glucose, Bilirubin, Coagulation profile, Ammonia).”  The key feature here is that the junior ICU trainee is expected to "describe the interpretation", not "interpret" or "apply clinically". CICM examiners expect the interpretation of deranged liver function tests only from their most senior trainees, and questions involving this are common in the Second Part Exam. 

So what does it mean to "describe the interpretation"? Is it different from an explanation of the tested biomarkers' normal function and the conditions under which they might be raised, which is already on offer in the Required Reading section for the Second Part exam? Reader, we are fortunate that the college examiners have favoured us with their thoughts on what the correct structure for this answer might look like. Though they did not reveal what exactly they wanted to the candidates writing the paper (because presumably a set of clear instructions would have given them an unfair advantage), they did include their preferred answer format in their comments on Question 7 from the second paper of 2021:

"...a definition of the variable to provide context, a normal value and the range of influences that effect the variable both related to liver function and or extrinsic to the liver ... whether the variable was sensitive to acute or chronic changes in liver function and which synthetic/metabolic component of the liver the variable represented"

Using this framework, this chapter focuses on "Albumin, Glucose, Bilirubin, Coagulation profile, Ammonia", and liver enzymes are left in the other chapter. That's how we are going to split this topic (but a summarised explanation of liver enzyme interpretation is also included here to simplify revision).

Additionally, here's a quick summary of frequently tested hepatic enzymes, to simplify revision:

It does not seem as if anybody has ever tried to write a single all-encompassing review article which covers each and every part of what the CICM syllabus document has asked for. Wherever one finds something that sounds suitable, the authors are often guilty of overfocusing on the liver enzymes. Of these, the best option would probably be Woreta & Alqahtani (2014) or Wolf (1999), the latter being the most comprehensive review.

Albumin levels to assess liver function

Albumin, discussed to the point of nauseating pedantry in another chapter, is notable as a test of liver function, because:

• It is produced exclusively by the liver, and
• It is easy to test for because it is by far the most numerous protein in the bloodstream (being produced at a rate of 10-15g/day)

Ergo:

• If the albumin level is low, the function of the liver may be impaired.

However:

• A normally functioning liver can decide to stop producing albumin in the context of a non-specific stress response, and the resulting drop in albumin would be viewed as a perfectly normal physiological reaction (thus albumin is sometimes referred to as a negative acute phase reactant)
• A normally functioning liver may choose to produce less albumin in order to preserve scarce amino acid resources, for example in the context of prolonged malnutrition, in which case the albumin will be low but the function of the liver will be normal.
• A poorly functioning liver may be producing a very small amount of albumin, but the patient may be severely dehydrated and the haemoconcentration effect may produce a normal albumin value on the blood test.
• Albumin may be lost by other mechanisms, such as nephrotic syndrome, which would drop albumin levels in the presence of a normal liver
• A patient's liver can fail acutely and totally, but the albumin level may remain normal for some time, as it has a substantial half-life and it will take some time for it to be depleted

Thus, albumin is not an especially sensitive or specific biomarker of liver function, and indeed is rarely used as such. Still, there is clinical value in measuring the albumin level for the assessment of liver disease. Specifically, it has prognostic value in chronic liver failure: for example, it is incorporated into the Child-Pugh scoring system, and adding it to the MELD score seems to improve its predictive value. This may be at least partly due to the relationship between albumin and nutrition, i.e. the association with poor outcomes might be mainly because it is low in malnoursihed patients, rather its association with liver synthetic function. This was demonstrated cleverly by Rothschild et al back in 1969: the rate of synthesis of albumin in nineteen cirrhotic subjects with ascites was depressed in seven, but stable in five and actually raised in another seven. 

Glucose levels to assess liver function

The liver is a major player on the arena of carbohydrate metabolism, as it is uniquely capable of storing glucose in the form of glycogen and fat, to release it later as the organism requires. It is therefore theoretically possible that the liver, if sufficiently damaged, will no longer be able to do this, and the organism will become hypoglycaemic. Moreover, it follows that the less remaining working liver tissue one has left, the worse the hypoglycaemia, and therefore the worse the prognosis of the liver failure. This was certainly the finding of Kaur et al (2013), for whom hypoglycaemia was an independent risk factor for mortality. The situation can get so bad that the brain is forced to survive by metabolising lactate (in 2014, Oldenbeuving et al reported a case of a paracetamol overdose patient whose BSl was 0.7 mmol/L and who who remained conscious - it is thought - purely because their lactate was 25mmol/L).

However, this sort of hypoglycaemia is really only seen in hyperacute fulminant liver failure. In chronic liver disease the more common observation is actually hyperglycaemia, with most cirrhosis patients manifesting a mildly diabetes-like glucose intolerance(Brown et al, 1967). There are several reasons for this; for example the liver plays a role in the metabolism of insulin (Duckworth et al, 1988), which means more insulin will be hovering around in the circulation. At the same time there is a change to a hypercatabolic starvation-like state, with much of the metabolic demand being satisfied by an increase in the plasma carriage of free fatty acids, which tends to produce insulin resistance. 

Bilirubin levels to assess liver function

Biliribin is a toxic haem metabolite which is eliminated exclusively by the liver, and this makes it a convenient biomarker of liver function in much the same way as we use creatinine. Unfortunately, unlike creatinine, the relationship is not completely direct. A patient may have a wildly elevated bilirubin for a variety of extrahepatic reasons, all of which are still bad but none of which are related to the function of the liver. Examples include:

• Intravascular haemolysis (eg. autoimmune haemolytic anaemia)
• Accelerated red cell phagocytosis (eg. the reabsorption of a large haematoma, or haemophagocytic syndrome)
• Extrahepatic biliary obstruction

So really, the only liver problems where bilirubin might be raised are:

• A failure to acquire bilirubin from the blood,
• A failure to conjugate bilirubin, or
• A failure to excrete conjugated bilirubin into the bile

And even the former may take a benign form, eg. in Gilbert syndrome, where the liver is otherwise totally normal and the elevation of unconjugated bilirubin can be safely ignored. To help the reader structure these possibilities in their own mind,  Ruiz et al (2021) summarise these scenarios best, using a familiar-looking structure:

• Pre-hepatic causes of hyperbilirubinemia (eg. haemolysis)
• Post-hepatic causes of hyperbilirubinemia (eg. obstruction)
• Hepatic causes of hyperbilirubinemia (all possible forms of live damage):
  • Failure of bilirubin uptake
    • Gilbert syndrome.
    • Drugs—for example, rifampicin
  • Failure of bilirubin conjugation
    • Gilbert syndrome.
    • Criggler-Najjar syndrome.
    • Physiological jaundice of the newborn
  • Failure of bilirubin excretion
    • Dubin-Johnson syndrome.
    • Rotor’s syndrome.
    • Sepsis
  • Failure of all 3 processes
    • Basically any destructive liver disease that results in hepatocyte death or dysfuction
    •  Ruiz et al list "viral hepatitis, alcoholic hepatitis, metabolic steatohepatitis, toxic hepatitis, Wilson disease, hemochromatosis, autoimmune hepatitis, α₁-antitrypsin deficiency, ischemic hepatitis, and Budd–Chiari syndrome", but there are probably one million others.

Within the context of known liver disease, bilirubin testing is also valuable for its prognostic implications.  Specifically, progressive liver damage in cirrhosis is accompanied by a progressive rise of baseline bilirubin. For its predictive value, bilirubin is incorporated into the Child-Pugh, CLIF-C ACLF and MELD scoring systems. 

Coagulation profile to assess liver function

Yes, the work of the coagulation cascade can be used as a reasonable measure of liver function, as the liver produces basically all of the important components of the clotting cascade, except for:

• Platelets 
• Factor III (tissue factor, produced in the vascular endothelium)
• Calcium (Factor IV)
• Factor VIII (antihaemophilic factor), of which a proportion is produced in the vascular endothelium
• Von Willebrand Factor (also produced in the endothelium)

All the others are synthesised in the liver, and most patients with liver disease have at least some sort of clotting dysfunction, although the dysfunction may not be related to insufficient clotting (as antithrombotic factors like proteins C and S are also underproduced). It is extremely difficult to track down a single resource which gives a breakdown of which factor does what in this scenario, which means the following table had to be cobbled together from pieces of Lechner et al (1977), Amitrano et al (2002), Agarwal et al (2013), Mammen (1992) and Trotter (2006).

Effect of Liver Disease on Clotting Cascade Proteins

Factor	Effect of liver disease
Factor I: fibrinogen	Only reduced in fulminant acute liver failure, or in advanced decompensated cirrhosis
Factor II: prothrombin	Reduced (because of reduced vitamin K)
Factor III: thromboplastin
Factor IV: calcium	Might even increase as the result of less albumin and therefore a larger fraction free calcium being available
Factor V: "proaccelerin"	Reduced
Factor VII: "proconvertin"	Reduced (because of reduced vitamin K)
Factor VIII: "Antihaemophilic factor"	Normal, or increased (especially in acute liver failure)
Factor IX: Plasma thromboplastin component	Reduced (because of reduced vitamin K)
Factor X: Stuart-Prower factor	Reduced (because of reduced vitamin K)
Factor XI: Plasma thromboplastin antecendent	Reduced in a minority of liver failure patients
Factor XII: Hageman's factor	Reduced in a minority of liver failure patients
Protein C	Reduced only in very advanced liver disease
Protein S	Reduced only in very advanced liver disease
Antithrombin	Reduced (by poor synthesis and increased consumption)
Tissue plasminogen activator (TPA)	Increased (because of decreased hepatic clearance)
Platelets	Decreased (because of increased destruction in the portally congested spleen, as well as reduced thrombopoietin secretion by the diseased liver)

Nobody could possibly be expected to remember all that; but fortunately it is a largely pointless exercise performed for completeness. The salient points of it can be summarised in terms of the net effect of liver dysfunction on the routine tests of coagulation, which makes for a much smaller table:

Effect of Liver Disease on Clotting Function

Effect of liver disease	Effect of this on tests of clotting function
Decreased intrinsic pathway protein synthesis	Increased APTT Prolonged R-time on TEG
Decreased synthesis of Vitamin K-dependent factors	Increased PT and INR Unreliable TEG effects
Decreased fibrinogen synthesis	Decreased MA on TEG Low fibrinogen levels, obvs
Decreased synthesis of anticoagulant factors (proteins C, S, antithrombin)	Unexpected prothrombotic tendency in spite of high APTT and INR A surprisingly normal-looking TEG  A potential for resistance to heparin
Decreased clearance of TPA	A propensity to fibrinolysis; increased LY30 on TEG

The bottom line here is that clotting function changes in liver disease are difficult to assay or predict because of the balance of prothrombotic and antithrombotic effects, and overall clotting function may be normal in the presence of wildly deranged coagulation tests. This has all sorts of surprising implications for the management of the liver disease patient. For example, one may have a patient with liver failure who has clearly very abnormal coagulation studies (eg. an INR of 3.0, and APTT of 50), but who is functionally prothrombotic, and who would still benefit from some sort of chemical DVT prophylaxis.

Ammonia level to assess liver function

Ammonia is produced as the end result of amino acid metabolism in the liver and various other tissues. It is interesting for the purposes of assessing liver function mainly because the liver is the host of urea cycle enzymes for which it is a substrate. The liver function being assessed here is therefore the elimination of ammonia, and indirectly the synthesis of urea; where a rising serum ammonia level is interpreted as evidence of liver dysfunction.

Of course, poor liver function is not the only way you could get an elevated ammonia level. It could also be the result of:

• Increased protein uptake and metabolism (eg. a preposterously protein-rich diet)
• Increased rates of protein catabolism (eg. starvation or haematological malignancy)
• Increased production by altered populations of gut organisms (eg. short gut syndrome)
• Decreased clearance by impaired urea cycle enzymes in an otherwise healthy liver (eg. in the context of a congenital enzymopathy 

And in those cases the ammonia level and the level of liver dysfunction would be clearly unrelated to one another. In which case ammonia would still probably be worth measuring, because it has its own toxicity, irrespective of how it is being generated or why it is raised. But this is probably outside the scope of this section, as we are mainly interested in the laboratory tests of liver function (for more information on the possible interpretation of a raised ammonia or on the pathways of its metabolism the reader is referred to Clay & Hainline, 2007). 

Ammonia measurements can be helpful in the following liver-related scenarios:

• Hepatic encephalopathy: ammonia levels are often ordered in hepatic encephalopathy, even though the actual level itself does not correlate especially well with the severity of encephalopathy, at least not in chronic liver disease; to the point where its frequent use for this purpose has been described as wasteful (Aby et al, 2021).  The only possible valid rationale for the serial sampling is that the contribution of ammonia to encephalopathy is not zero, and in any case many of the therapies which are used to treat hepatic encephalopathy will also have the effect of lowering serum ammonia - so it can be used to track the progress of therapy. Or, it may ben
• Cerebral oedema in acute liver failure: The sudden loss of liver function can lead to an abrupt increase in ammonia levels which would then bring about osmotic cerebral oedema, as it is captured and converted to osmotically active glutamine by astrocytes. This means patients with acute liver failure often require haemodiafiltration to remove ammonia, and serial ammonia levels are used to adjust the dose of dialysis.
• Acquired or congenital urea cycle defects: where one might have some inborn error of metabolism, or where such an error has occurred as the result of drug effects (for example sodium valproate overdose), the use of serum ammonia measurements is fairly routine, as the ammonia causes a lot of the harm, and removing the ammonia is often as important as addressing the primary cause.