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, with no past paper SAQs to refer to, it is ever so difficult to guess what the curriculum authors had intended. But at least they have left us with a prescriptive list of variables to cover. This chapter, therefore, 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).

Synthetic and metabolic products of the liver

  • Albumin as a biomarker of liver function
    • Ubiquitous protein produced almost exclusively by the liver
    • Depressed albumin levels can a failure of synthetic function
    • Albumin can also be depressed in other circumstances (starvation, malnutrition, stress, critical illness, protein loss via nephrotic syndrome)
    • A long half life means albumin levels can remain normal for some time even as hepatic synthesis has stopped
    • Albumin has has prognostic relevance in liver disease (eg. Child-Pugh score)
  • Glucose as a biomarker of liver function
    • The liver stores glucose as glycogen and triglycerides, and is able to mobilise glucose into the bloodstream as needed
    • Hypoglycaemia is often associated with acute liver failure
    • In chronic liver failure, hyperglycaemia is often seen, due to decreased hepatic insulin clearance and increased peripheral insulin resistance
  • Bilirubin as a biomarker of liver function
    • Bilirubin is cleared exclusively by the liver via the biliary route, making it a suitable biomarker of hepatobiliary function
    • Hepatocytes conjugate bilirubin:
      • An excess of unconjugated bilirubin can be a sign of poor metabolic liver function
    • Conjugated bilirubin is eliminated in the bile:
      • Raised conjugate bilirubin levels can be a sign of biliary obstruction or canalicular dysfunction
    • Bilirubin can also be raised independently of hepatic dysfunction, eg:
      • where it is overproduced (haemolysis)
      • where there is a harmless defect of its elimination (eg. Gilbert syndrome).
    • A raised bilirubin has prognostic relevance (eg. Child-Pugh score)
  • Coagulation function as a surrogate measure of synthetic liver function
    • ​​​​​​​The liver produces all clotting factors other than Factor III (tissue factor), Factor IV (calcium), Factor VIII and VWF
    • Liver disease can decrease or increase clotting function:
      • Decreased clotting function by decreased synthesis of clotting factors and decreased clearance of tPA
      • Increased clotting function by decreased synthesis of antithrombotic factors (eg. proteins C and S)
    • The net effect of poor synthetic liver function is complex and not uniformly antithrombotic
  • Ammonia as a biomarker of liver function
    • Ammonia is the product of amino acid catabolism and a substrate for the urea cycle enzymes which reside in the liver
    • A raised ammonia level suggests poor metabolic liver function, a dysfunction of urea cycle enzymes, or increased protein turnover
    • A raised ammonia level is sometimes used to assess the response to therapy for hepatic encephalopathy 
    • Uncontrolled hyperammonaemia can give rise to cerebral oedema

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

Hepatic enzymes used as biomarkers of liver function

  • Aspartate aminotransferase (AST)
    • Ubiquitous enzyme which converts oxaloacetate into aspartate.
    • Found in many extrahepatic tissues, including cardiac and skeletal muscle 
    • An early but nonspecific biomarker of hepatocellular injury 
  • Alanine transaminase (ALT)
    • Kreb cycle enzyme found in hepatocytes; half life 50 hrs
    • Released later, after AST, in hepatocellular injury
  • γ-glutamyl transferase (GGT)
    • ​​​​Induceable microsomal enzyme; transfers γ-glutamyl off glutathione
    • Found in numerous tissues (renal tubules, liver, biliary tract, pancreas)
    • A highly non-specific marker of biliary tract disease
  • Alkaline phosphatase (ALP)
    • A hydrolase which removes the phosphate group from various molecules
    • Found in all tissues (eg. bone, small intestine, leukocytes and the placenta)
    • A non-specific marker of biliary tract stress
  • Lactate dehydrogenase (LDH)
    • C​​​​atalyses the conversion of lactate acid to pyruvate
    • Found in essentially all cells
    • A non-specific marker of liver damage

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-PughCLIF-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

Decreased fibrinogen synthesis

  • Decreased MA on TEG
  • Low fibrinogen levels, obvs

Decreased synthesis of anticoagulant factors (proteins C, S, antithrombin)

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. 

References

Woreta, Tinsay A., and Saleh A. Alqahtani. "Evaluation of abnormal liver tests." Medical Clinics 98.1 (2014): 1-16.

Wolf, Paul L. "Biochemical diagnosis of liver disease." Indian Journal of Clinical Biochemistry 14.1 (1999): 59-90.

Chen, Chiung-Yu, et al. "The value of serum ischemia-modified albumin for assessing liver function in patients with chronic liver disease." Clinical chemistry and laboratory medicine 49.11 (2011): 1817-1821.

Carvalho, Joana R., and Mariana Verdelho Machado. "New insights about albumin and liver disease." Annals of hepatology 17.4 (2018): 547-560.

Myers, Robert P., et al. "Revision of MELD to include serum albumin improves prediction of mortality on the liver transplant waiting list." PloS one 8.1 (2013): e51926.

Whicher, J., and C. Spence. "When is serum albumin worth measuring?." Annals of clinical biochemistry 24.6 (1987): 572-580.

Rothschild, Marcus A., et al. "Albumin synthesis in cirrhotic subjects with ascites studied with carbonate-14 C." The Journal of Clinical Investigation 48.2 (1969): 344-350.

Duckworth, William C., Frederick G. Hamel, and Daniel E. Peavy. "Hepatic metabolism of insulin." The American journal of medicine 85.5 (1988): 71-76.

Brown, Henry, et al. "Ammonium and glucose metabolism in liver failure." JAMA 201.11 (1967): 873-874.

Nolte, W., H. Hartmann, and G. Ramadori. "Glucose metabolism and liver cirrhosis." Experimental and Clinical Endocrinology & Diabetes 103.02 (1995): 63-74.

Kaur, Sharandeep, et al. "Etiology and prognostic factors of acute liver failure in children." Indian pediatrics 50.7 (2013): 677-679.

Oldenbeuving, G., et al. "A patient with acute liver failure and extreme hypoglycaemia with lactic acidosis who was not in coma: causes and consequences of lactate-protected hypoglycaemia." Anaesthesia and intensive care 42.4 (2014): 507-511.

Ruiz, Armando Raúl Guerra, et al. "Measurement and clinical usefulness of bilirubin in liver disease." Advances in Laboratory Medicine/Avances en Medicina de Laboratorio 2.3 (2021): 352-361.

Lechner, K., H. Niessner, and E. Thaler. "Coagulation abnormalities in liver disease." Seminars in thrombosis and hemostasis. Vol. 4. No. 01. Copyright© 1977 by Thieme Medical Publishers, Inc., 1977.

Agarwal, Banwari, et al. "Evaluation of coagulation abnormalities in acute liver failure." Journal of hepatology 57.4 (2012): 780-786.

Mammen, Eberhard F. "Coagulation abnormalities in liver disease." Hematology/oncology clinics of North America 6.6 (1992): 1247-1257.

Trotter, James F. "Coagulation abnormalities in patients who have liver disease." Clinics in liver disease 10.3 (2006): 665-678.

Amitrano, Lucio, et al. "Coagulation disorders in liver disease." Seminars in liver disease. Vol. 22. No. 01. Copyright© 2002 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel.:+ 1 (212) 584-4662, 2002.

Ninan, Jacob, and Leonard Feldman. "Ammonia levels and hepatic encephalopathy in patients with known chronic liver disease." Journal of hospital medicine 12.8 (2017): 659-661.

STAHL, JULES. "Studies of the blood ammonia in liver disease: its diagnostic, prognostic, and therapeutic significance." Annals of internal medicine 58.1 (1963): 1-24.

Aby, Elizabeth, Andrew PJ Olson, and Nicholas Lim. "Serum ammonia use: unnecessary, frequent and costly." Frontline Gastroenterology (2021).