Postscript

For case study, click here.

Introduction

Pre-endoscopic differentiation between upper and low gastrointestinal (GI) bleeding helps to inform and prioritize next steps in the diagnostic pathway.
An elevated blood urea nitrogen (BUN) to creatinine ratio has been shown to increase the likelihood of an upper GI source of blood loss.
The aim of this Tweetorial is to provide a conceptual framework for understanding this observation.
We will consider the following:
- Blood in the upper GI tract is degraded to its constituent parts, including hemoglobin-derived amino acids.
- These amino acids are absorbed by the intestinal epithelial cells.
- Once absorbed, some amino acids are used for intestinal needs, the rest are transported to the liver.
- In the liver amino acids undergo one of several fates:
  - They may be exported to the systemic circulation and transported for use by other tissues.
  - They may be used to synthesize new protein.
  - They may be broken down into nitrogen-containing amine groups and carbohydrate skeletons.
    - The amine groups (ammonia) are converted to urea, which is then excreted by the kidney. Hence, blood urea concentration may increase in the setting of an upper GI bleed.
    - The carbohydrate skeletons are used to generate energy and to synthesize glucose and fatty acids.
- Creatinine concentration is unchanged since it reflects muscle mass, not dietary protein intake.
- Thus, the BUN to creatinine ratio is increased in upper GI bleeding.
- Blood has low biological value (poor nutritional availability for use by the body).
- Proteins of low biological value give rise to higher ammonia and urea production compared with balanced proteins. Thus, blood in the stomach will cause a greater increase in the BUN compared with an isonitrogenous protein meal.

Proteins and amino acids – overview

Proteins consist of chains of amino acids (amino acids serve as the building blocks for proteins).
There are a total of 20 amino acids:
- 12 amino acids can be obtained from diet or synthesized in the body (non-essential).
- 8 amino acids cannot be synthesized in the body; they must be obtained by diet (essential).
Normal protein balance in adult humans:
- Healthy diet includes 75-100 grams of protein per day:
- Major sources of dietary proteins include muscle, milk, egg and plant proteins.
- 300-500 grams of new protein are synthesized each day for maintenance.
- 300-500 grams of protein are degraded each day to their constituent amino acids.
Protein functions include:
- Catalyzing metabolic reactions
- DNA replication
- Responding to stimuli
- Providing structure to cells and organisms
- Transporting molecules from one location to another

Amino acid metabolism in GI tract

Ingested proteins are subjected to complex changes during ingestion, digestion and absorption.
Digestion:
- Proteins are hydrolyzed by proteinases, especially gastric pepsin.
- Resulting peptides may be further hydrolyzed by peptidases present in the pancreatic secretions.
- Resulting small peptides are further digested by brush-border peptidases at the surface of the epithelial cells to produce free, di-, tri- and oligo-amino acids, which are absorbed.
Absorption:
- Amino acid transport occurs in a sodium- and gradient-dependent manner across the brush border membrane of intestinal epithelial cells.
- The transport systems are specific to groups of amino acids rather than to individual amino acids.
- In addition to the rate of digestion and absorption of amino acids, utilization of amino acids by splanchnic tissues (portal-drained viscera) is a major factor determining the bioavailability of amino acids for other tissues and organs.
- Splanchnic tissues retain between 20–50% of the dietary intake of specific essential amino acids.

Amino acid metabolism in liver

Amino acids are transported from intestine to liver via the portal vein.
The liver can be considered as a gatekeeper that determines whether specific amino acids are delivered systemically to body tissues or are taken up by the liver:
- Virtually all amino acid transporters are expressed in the liver.
- Despite the presence of cognate receptors, the branched-chain amino acids (BCAAs; valine, leucine, and isoleucine) are not taken up in the liver.
- Alanine accounts for ~50% of total hepatic amino-acid uptake.
Once in the hepatocyte, amino acids are:
- Released into blood for use by other tissues, especially skeletal muscle.
- Used as building blocks for protein synthesis (anabolism) in the liver, including albumin, lipoproteins, transferrin, and retinol-binding protein.
- Metabolized/catabolized to yield:
  - NH3 (ammonia):
    - Release mediated by:
      - Transamination step:
        
        Transfer of amino group from one amino acid to a keto acid (for example, alpha-ketoglutarate) to form another amino acid (for example, glutamate), and a second keto acid.
      - Deamination step (removal of amino groups from amino acids to form ammonia):
        
        Removal of alpha amino group (for example from glutamate, which serves as a collecting station for amine groups) in form of NH3 (ammonia).
        
        Ammonia is converted to urea, which is water soluble and non-toxic.
        
        This process is needed for getting rid of nitrogen from the animal’s body.
  - Carbon skeletons (or carbon backbone remaining from deamination of amino acids), which can then be:
    - Metabolized to:
      - Pyruvate
      - Intermediate of tricarboxylic acid (TCA) cycle
      - Acetyl CoA
      - Acetoacetyl CoA
    - Used to:
      - Generate energy via TCA.
      - Synthesize glucose (from “glucogenic” amino acids), which can used used for energy needs in liver, stored as glycogen or released into the circulation for use by other tissues.
        
        Breakdown of 100 g of proteins yields:
        
        60 g of glucose/day
        
        400 kcal/day
      - Synthesize fatty acids (from “ketogenic” amino acids, including lysine or leucine).

Schematic of an amino acid. Amino acids have a two-carbon bond. One of the carbons is part of a group called the carboxyl group (COO-). A carboxyl group is made up of one carbon (C) and two oxygen (O) atoms. The second carbon is connected to the amino group. Amino (or amine) means there is an NH2 group bonded to the carbon atom. The side groups are what make each amino acid different from the others. Catabolism of amino acids involves the removal of the amino group, which is ultimately converted to urea, leaving behind a carbohydrate skeleton which is used for energy production and synthesis of glucose and fatty acids.

Amino acids are absorbed by intestinal epithelial cells, transported to the liver where some (but not all) are taken up by hepatocytes. In hepatocytes, the amino acid may be used for protein synthesis or it may be catabolized to ammonium and carbohydrate skeletons.

Urea cycle

The ammonia liberated from amino acid degradation is toxic to the central nervous and needs to be excreted or detoxified.
Detoxification of ammonia to form urea is mediated by the urea cycle.
The urea cycle:
- Is a nitrogen clearance system.
- Is limited to the human liver and to a lesser extent the kidneys (two key enzymes, carbamyl phosphate synthetase and ornithine transcarbamylase limited exclusively to those tissues).
- Comprises a series of five enzyme reactions, some occurring in the mitochondria and others in the cytosol.
In the first step, ammonia, which is released from amino acids via stepwise transamination (transfer of amino group to glutamate) and deamination (removal of amino group from glutamate), combines with CO2 to form carbamoyl phosphate via the rate limiting enzyme carbamoyl phosphate synthase.
The urea cycle ultimately results in the formation of urea which is excreted by the kidneys.
Hyperammonemia develops if the urea cycle cannot control the ammonia load. Causes include:
- Genetic (urea cycle disorders [UCD])
  - Result from genetic mutations causing defects in the metabolism of the extra nitrogen produced by the breakdown of protein and other nitrogen-containing molecules.
- Acquired
  - Acute or chronic liver failure – diminished hepatic urea synthesis capacity, leading to an impaired capacity to detoxify ammonia.
  - Vascular bypass of the liver
  - Drug effects:
    - Valproic acid treatment
    - L-asparaginase treatment
  - Nitrogen overload of the system from:
    - Massive hemolysis (such as large bone fracture or trauma)
    - Total parenteral nutrition
    - Protein catabolism from starvation or bariatric surgery
    - Post partum stress
    - Heart Lung Transplant
    - Renal Disease
    - Gastrointestinal bleeding

Another perspective of the urea cycle. From Journal of Intensive Care 2014, 2:22.

About urea

Urea (60 daltons) is the principal nitrogenous waste product of metabolism and is generated from protein breakdown (principal nitrogenous end product of protein and amino acid catabolism).
More than 99% of urea synthesis occurs in the liver; its primary source is dietary protein.
The normal subject on a 70 g protein diet produces about 12 g of urea each day.
A small amount (<10%) of urea is eliminated via sweat and the gut.
The rest (about 10 gm each day) is freely filtered at glomerulus.
Urea is both reabsorbed and secreted in the kidney:
- Some urea is reabsorbed from the filtrate, depending on factors such as the state of hydration and renal blood flow.
  - At high urine flow rates (greater than 2 ml/min), 40% of the filtered load is reabsorbed.
  - At flow rates lower than 2 ml/min, reabsorption may increase to 60%.
- Urea is also secreted by renal tubules.
- The net effect of these two processes results in around 30-50% of the filtered urea appearing in urine.
Around the world, blood urea levels are expressed in different ways:
- In the United States only the nitrogen component of urea (the blood urea nitrogen [BUN]) is measured):
  - Despite its name, this assay uses plasma or serum, not whole blood.
  - Reported as mg/dL.
- In other parts of the world, the whole urea molecule (not just the nitrogen part of the molecule) is assayed and reported (in SI units, mmol/L)
- Since BUN reflects only the nitrogen content of urea (MW 28) and urea measurement reflects the whole of the molecule (MW 60), urea is approximately twice (60/28 = 2.14) that of BUN.
  - To convert BUN (mg/dL) to urea (mmol/L): multiply by 10 to convert from /dL to /L and divide by 28 to convert from mg BUN to mmol urea, i.e. 10/28 = 0.357.
  - So the conversion factor is 0.357: BUN mg/dL multiplied by 0.357 = urea (mmol/L), Urea (mmol/L) divided by 0.357 = BUN (mg/dL).
  - Approximate reference (normal) range:
    - Serum/plasma urea 2.5-7.8 mmol/L.
    - Serum/plasma BUN 7.0-22 mg/dL.
Causes of increased BUN:
- Increased urea production:
  - Physiological
    - Increased dietary protein
    - Ageing
  - Pathological:
    - Gastrointestinal hemorrhage (blood in the gut is effectively a high-protein meal).
    - Increased protein catabolism:
      - Trauma
      - Major surgery
      - Starvation
      - Severe infection
      - Drugs
    - Decreased renal perfusion due to low circulatory states:
      - Heart failure
      - Dehydration
      - Hypovolemic shock
- Decreased urea elimination
  - Acute kidney injury
  - Chronic kidney disease

Linear relationship between protein content of diet and blood urea concentration in rats. Br. J. Nutr. (1970), 24, 983

About creatinine

Creatinine (113 daltons) is a breakdown product of creatine phosphate from muscle.
It is released at a constant rate by the body (production reflects muscle mass).
The kidneys are normally the only route of excretion.
It is freely filtered by the glomerulus, but unlike urea, it is not reabsorbed or affected by urine flow rate.
A small amount is secreted by proximal tubules.
The total creatinine excretion in a normal man averages 14 to 26 mg/kg/day, and in a normal woman 11 to 20 mg/kg/day.
Increased plasma creatinine is almost invariably a consequence of reduced GFR and therefore has a renal cause.
Unlike urea, creatinine is largely unaffected by gastrointestinal bleeding or by catabolic factors such as fever and steroids.

About urea: creatinine ratio

Urea, but not creatinine levels are influenced by non-renal conditions.
Thus, interpretation of increased plasma urea is often aided by simultaneous measurement of creatinine and calculation of the urea: creatinine ratio in order to establish a renal or non-renal cause.
As mentioned above, in the US and a few other countries, urea concentration is expressed as the nitrogen content of urea (MW 28) and reported as blood urea nitrogen (BUN) in non-SI units (mg/dL).
In all other parts of the world, urea results are expressed as the whole urea molecule (MW 60) and reported as urea in SI units (mmol/L).
The non-SI ratio (BCR) is BUN (mg/dL) / plasma creatinine (mg/dL):
- The reference range is around 8-15.
- The most commonly used cut-off value to define increased BCR is 20.
The SI ratio (UCR) is plasma urea (mmol/L) / (plasma creatinine (μmol/L) divided by 1000):
- The factor of 1000 is needed to convert creatinine result from μmol/L to mmol/L, the urea unit of measurement.
A solution to this interpretative problem is to convert SI results for urea and creatinine to non-SI results before calculating the ratio.
Causes of increased plasma urea and normal creatinine (elevated urea/creatinine ratio):
- Dehydration
- Heart failure (without renal involvement)
- Gastrointestinal bleed
- High-protein diet
- Catabolic state due to
  - trauma
  - severe infection
  - starvation
  - corticosteroid drugs
Causes of increased ratio:
- Increased BUN reabsorption.
- GI bleed or increased dietary intake.

GI bleeding

The problem:
- In the United States, about 400,000 hospitalizations occur each year with a principal diagnosis of gastrointestinal bleeding (GIB).
- Patients hospitalized with a upper GIB (UGIB) have a mortality of 4.5% to 8.2%, and similar patients with a lower GIB (LGIB) have a mortality of 3.0% to 8.8%.
- Identification of a UGIB (proximal to the ligament of Treitz) vs LGIB (distal to the ligament of Treitz) is critical to performing an effective, efficient evaluation and may be difficult even for an experienced clinician.

From JAMA. 2012 Mar 14;307(10):1072-9.

A literature search identified 25 studies that compared predictors of upper and lower GIB:
- The following clinical parameters were predictive of an UGIB:
  - A prior history of UGIB.
  - Age less than 50 years.
  - Use of warfarin.
  - A history of passing black or tarry stool.
  - Nasogastric lavage with clear evidence of blood in the aspirate (blood or coffee grounds grossly present).
  - An increased serum urea nitrogen: creatinine ratio (> 30 results in a homogeneous diagnostic odds ratio (I2=17%, P=.31) with a summary LR of 7.5 (95% CI, 2.8-12.0).
  - Increasing severity of anemia.

JAMA. 2012 Mar 14;307(10):1072-9

Cohort of 76 adult patients with upper GI bleeding:
- Patients were divided into two groups:
  - BUN higher than the normal limit (21.0 mg/dL) (H).
  - BUN lower than the normal limit (L).
- BUN and BUN/Cre ratio were higher in the H group than in the L group.
- White blood cell count was higher and hemoglobin was lower in the H group.
Cohort of 124 adult patients:
- A total of 63 (51%) presented with blood in stool and 53 (43%) with bloody emesis; 8 (6%) had blood in both emesis and stool.
- A total of 31 (25%) patients had a lower GI bleed, 88 (70%) had an upper, and 5 (4%) had both upper and lower bleeding sources.
- The mean BUN level was 24 mg/dL, the mean Cr level 1.03 mg/dL, and the mean BUN/Cr ratio was 24.
- Upper GI bleeding was significantly correlated with:
  - Age younger than 50 (P = .01)
  - Male gender (P = .01; odds ratio, 3.13).
- Taking into account age and gender, the BUN/Cr ratio correlated significantly with an upper GI source of bleeding (P = .03), with a ratio greater than 36 having a sensitivity of 90% and a specificity of 27%.
Cohort of 11 children with 16 episodes of upper GI bleeding and 49 with lower GI bleeding:
- Significant difference between the two GI bleeding groups with regard to BUN/Cr ratio (p less than 0.001).
- BUN/Cr ratio 30 or above, the specificity of upper GI bleeding was 98% with a sensitivity of 68.8%
- A linear relationship was found between the BUN/Cr ratio and delta Hb (delta Hb = 0.08 x BUN/Cr +/- 0.8 g/dl) for bleeding originating from the upper GI tract
Cohort of 621 patients with upper and lower GI bleedings:
- BUN/Cr ratio > 35
  - Sensitivity 19.63% (95%CI: 16.69 – 23.45)
  - Specificity 90.16% (95%CI: 83.11 – 94.88)
  - Positive predictive values 89.09 (95%CI: 81.35 – 93.98)
  - Negative predictive values 21.53 (95%CI: 18.09 – 25.39)
Systematic review of the literature to find studies reporting the diagnostic precision of the BUN-to-creatinine ratio in patients with GI bleeding without hematemesis:
- Elevated BUN-to-creatinine ratio significantly increase the probability of finding an upper GI source.
- However, the sensitivity for a diagnosis of upper GI bleeding is low (negative likelihood ratio of 0.6).

Scoring systems for upper GI bleeding

Endoscopic hemostasis represents the cornerstone of UGIB treatment, and several scores have been developed for the prediction of clinical intervention. The most well established and commonly used pre-endoscopic scores are:
- Glasgow-Blatchford score (GBS):
  - A pre-endoscopic score developed to predict a composite of clinical intervention or death.
  - Contains the following parameters:
    - Initial hemoglobin levels
    - Urea:
      - Interestingly, the urea/creatinine ratio did not add further discriminatory power to the absolute urea level.
      - Authors state: “we noted that raised blood urea was a better marker of risk in acute upper-gastrointestinal hemorrhage than this ratio”.
    - Blood pressure
    - Pulse
    - Known syncope
    - Melena
    - Liver or cardiac failure.

- Pre-endoscopic or “admission” Rockall score (RS) – designed to predict death.
- AIMS65 score – designed to predict death.
Up-to-Date:
- Because blood is absorbed as it passes through the small bowel and patients may have decreased renal perfusion, patients with acute upper GI bleeding typically have an elevated blood urea nitrogen (BUN)-to-creatinine or urea-to-creatinine ratio.
- Values >30:1 or >100:1, respectively, suggest upper GI bleeding as the cause.
- The higher the ratio, the more likely the bleeding is from an upper GI source.
What do the guidelines say?
- Diagnosis and management of acute lower gastrointestinal bleeding: guidelines from the British Society of Gastroenterology: “Findings that are suggestive of an upper GI source of bleeding are brisk rectal bleeding and hemodynamic compromise, a past medical history of peptic ulcer disease, portal hypertension, elevated blood urea/creatinine ratio and patients with risk factors for UGIB, such as the use of antiplatelet drugs.”
- The ACG 2016 clinical guidelines for treating lower gastrointestinal (GI) tract bleeding: “An elevated blood urea nitrogen-to-creatinine ratio also suggests an UGIB source (likelihood ratio of UGIB with ratio >30:1 is 7.5),10 whereas red blood and clots are unlikely to be from an upper gastrointestinal source (likelihood ratio 0.05).

Upper GI bleeding is akin to a low biologic value “blood meal”

500 ml of whole blood is equivalent to 100 g protein (assuming a plasma protein level of 7.5 g/dl and a hemoglobin of 15 g/dl).
98% of the erythrocyte protein is hemoglobin:
- A hemoglobin molecule is made up of four polypeptide chains, two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each.
- Each red blood cells is packed with ~260 million hemoglobin molecules.
- Thus each red blood cell contains 260 million hemoglobin molecules x 574 amino acids per molecule of hemoglobin = 149 billion amino acids.
- So, an upper GI bleed of 250 ml blood (about 1 trillion RBCs) will yield 1.5 x 10²³ amino acids!
The hemoglobin molecule:
- Is totally devoid of the amino acid isoleucine. Plasma also contains little isoleucine, thus whole blood is severely deficient in the essential amino acid, isoleucine.
- Contains large amounts of the two other branched chain amino acids (BCAAs), valine and leucine.
The nutritional value of blood protein is poor. It is considered to have low biological value.
Proteins with poor biological value result in far greater increases in blood urea nitrogen.

Medical Hypotheses (1999) 52(6), 515–519

Experimental model

Pig model of intraluminal bleeding:
- Pigs were fed a standard pig meal or an isonitrogenous amount of bovine whole blood (400 ml).

- In the fasting state, arterial urea levels (2,320 +/- 353 moles per liter) always exceeded portal values (2,210
  +/- 291 moles per liter).
- After the standard meal, only a slight increase in systemic levels was observed (3,565 +/- 330 moles per liter), whereas porta-arterial differences were zero or stayed negative.
- After the isonitrogenous blood meal, a significant rise in systemic urea levels was noted (6,225 +/- 558 pmoles per liter).

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