Bilirubin, a byproduct of red blood cell breakdown, is derived from the heme in hemoglobin and excreted in the bile. Elevated levels in the blood are associated with liver or gallbladder disorders, inflammation, increased RBC breakdown, and certain medications. Determining whether the increase in bilirubin is due to the conjugated or unconjugated form will help identify the cause of the increase. Low total bilirubin has been closely associated with cardiovascular disease, stroke, metabolic syndrome, and diabetic peripheral neuropathy. Elevated total bilirubin can be seen with liver and gallbladder disorders, inflammation, and certain medications.
Standard Range: 0.20 - 1.20 mg/dL (3.42 - 20.52 umol/L)
The ODX Range: 0.5 – 0.90 mg/dL (8.55 – 15.39 umol/L)
Low total bilirubin is associated with increased cardiovascular risk, systemic inflammation (Akboga 2015), oxidative stress (Suh 2018), stroke (Zhong 2019, Perlstein 2008), increased carotid intima-media thickness, arterial stiffness, diabetic peripheral neuropathy (Kim 2015), and metabolic syndrome (Shiraishi 2019, Giral 2010). Drugs that may cause decreased total bilirubin levels include caffeine, barbiturates, penicillin, and large doses of salicylates (Pagana 2021).
High total bilirubin is associated with liver dysfunction, hepatitis, jaundice, inflammation, obstruction of extrahepatic ducts, RBC hemolysis (Pagana 2021), cholecystitis, and common bile duct stones (Gillaspie 2019, Zeng 2020).
Several drugs may increase total bilirubin levels including allopurinol, antibiotics, antimalarials, ascorbic acid, cholinergics, codeine, dextran, diuretics, epinephrine, methyldopa, MAOs, morphine, oral contraceptives, salicylates, steroids, sulfonamides, theophylline, vitamin A and large doses of nicotinic acid (Pagana 2021).
Bilirubin is a compound primarily derived from the breakdown of the hemoglobin heme into biliverdin, which is then converted to bilirubin. Heme from myoglobin, cytochromes, catalase, and ineffective erythropoiesis can also contribute to circulating bilirubin (Singh 2021).
Though ultimately excreted from the body and previously considered a waste product, bilirubin is a potent antioxidant and anti-inflammatory compound. It can help prevent atherosclerosis by inhibiting the proliferation of vascular smooth muscle cells, decreasing the risk of endothelial dysfunction, and reducing oxidation, especially of LDL-C (Suh 2018).
Bilirubin initially circulates in the blood in a lipophilic unconjugated form called indirect bilirubin, which usually accounts for 70-85% of total bilirubin (TBIL). Indirect bilirubin is delivered to the liver, where it is conjugated with glucuronic acid to become direct (conjugated) bilirubin, which is water-soluble. Liver cells then release direct bilirubin into the bile to be excreted via the GI or urinary tract. Excess circulating bilirubin can cause discoloration of the skin and jaundice, which can occur if total bilirubin is above 2.5 mg/dL (43 umol/L) or if more than 50% of total bilirubin in circulation is conjugated. It is essential to determine if the excess bilirubin is conjugated or unconjugated to identify the cause of the elevation. Excess conjugated direct bilirubin (DBIL) will have a cause outside of the liver, while excess unconjugated indirect bilirubin (IBIL) will usually mean liver dysfunction (Pagana 2021). If elevations in AST and ALT accompany jaundice, then liver damage is likely, whereas accompanying elevations in ALP and GGT suggest cholestasis (Singh 2021).
Serum total bilirubin will be markedly elevated with acute cholecystitis, e.g., 2.29 mg/dL (39.17 umol/L), and even higher with common bile duct stones, e.g., 3.74 mg/dL (63.7 umol/L). Researchers recommend conducting immediate imaging studies to identify common bile duct stones early with an elevated TBIL instead of simply monitoring levels and delaying intervention (Gillaspie 2019). It is important to note that total bilirubin will be increased in the event of RBC hemolysis, which can be caused by oxidative stress (Pagana 2021).
Traditionally, bilirubin was assessed primarily to identify liver and gallbladder issues. However, emerging research identifies it as a valuable biomarker for evaluating cardiovascular risk. Bilirubin has antioxidant and anti-inflammatory benefits and may be protective against cardiovascular disease, including preventing the oxidation of LDL cholesterol. Meta-analysis of 12 prospective studies supports this premise, as does the PREVEND population study of 7,222 non-CVD subjects. Findings suggest that CVD incidence decreases by 10% with a 2-fold increase in baseline TBIL, especially levels obtained during fasting, which increases circulating TBIL. The median TBIL in the study was 0.4 mg/dL (7 umol/L) (Kunutsor 2015).
A retrospective cross-sectional study found significantly lower TBIL and systemic inflammation in those with stable coronary artery disease versus controls. In the study, lower TBIL was significantly correlated with increasing severity of CAD, CRP, NLR, MPV, and RDW. The most severe CAD was associated with a TBIL of 0.53 mg/dL (9.07 umol/L), mild CAD was associated with 0.6 mg/dL (10.26 umol/L), and healthy controls maintained a TBIL of 0.65 mg/dL (11 umol/L) (Akboga 2015).
A meta-analysis of 20 studies confirms the association between higher TBIL and reduced long-term risk of CVD. However, researchers note a transient spike in TBIL associated with short-term mortality and MACE in hospitalized MI patients, an effect likely due to acute damage to cardiac tissue (Lan 2019).
One prospective study of 12,097 healthy subjects found a U-shaped association between incident coronary heart disease and TBIL. The lowest risk was associated with the third quintile of 0.7-0.82 mg/dL (12-14 umol/L). Researchers attribute the cardioprotective effect of higher TBIL to a relative increase in IBIL, whereas a relative rise in DBIL was associated with increased CAD risk (Lai 2018).
One retrospective cohort study of 8,992 middle-aged Japanese individuals found that a higher TBIL of 0.7 mg/dL (12 umol/L) or higher was associated with a significantly lower incidence of metabolic syndrome. Researchers note that protection from metabolic syndrome may be attributed to bilirubin’s significant antioxidant capacity. This includes its ability to protect cells from a 10,000-fold increase in exposure to hydrogen peroxidase (Shiraishi 2019).
The association between low TBIL, CVD, and metabolic syndrome was explored in a prospective study of 8,844 Korean individuals evaluated for new-onset CVD. The incidence of cardiometabolic disease and death increased as TBIL decreased. The lowest TBIL, below 0.44 mg/dL (7.53 umol/L), was associated with a significantly increased incidence of CVD, CVD deaths, metabolic syndrome, and the components of metabolic syndrome compared to the highest tertile of TBIL above 0.63 mg/dL (10.78 umol/L). Baseline TBIL was inversely correlated with abdominal obesity, insulin resistance, diabetes, and hypertension, conditions previously associated with decreasing TBIL. Researchers note that a low TBIL may be due to a decrease in the antioxidant heme oxygenase enzyme or a relative increase in oxidative stress that consumes endogenous antioxidants (Suh 2018).
A systematic review and meta-analysis of 11 observational studies suggest a neuroprotective effect of bilirubin. Research supports an inverse relationship between TBIL levels and the risk of stroke, including ischemic stroke, especially in males (Zhong 2019). In one of the studies reviewed comprising 13,214 participants, mean TBIL was significantly lower with a history of stroke at 0.63 mg/dL (12.1 umol/L) versus 0.71 mg/dL (10.8 umol/L) in those with no stroke history. In general, a review of NHANES data suggests that a 0.1 mg/dL (1.71 umol/L) increment increase in TBIL may reduce the risk of stroke by 9% and the risk of adverse stroke outcome by 10%. Subjects with a median TBIL of 0.9 mg/dL (15.4 umol/L) had the lowest prevalence of stroke in general (Perlstein 2008).
Research suggests that bilirubin protects against microvascular disease, including diabetic retinopathy, neuropathy, and cardiovascular autonomic neuropathy. A retrospective analysis of 1,327 T2DM patients found that those with diabetic peripheral retinopathy (DPN) had a mean TBIL of 0.65 mg/dL (11.12 umol/L). At the same time, those without DPN maintained a mean TBIL of 0.76 mg/dL (13 umol/L). Researchers conclude that a TBIL cut-off of 0.55 mg/dL (9.41 umol/L) or below predicts diabetic peripheral neuropathy (Kim 2015).
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