Poor diagnostic accuracy of a single fasting plasma citrulline concentration to assess intestinal energy absorption capacity

Circulating Apolipoprotein B-48 as a Biomarker of Parenteral Nutrition Dependence in Adult Patients with Short Bowel Syndrome

Short bowel syndrome (SBS) is a rare but serious condition that may lead to chronic intestinal failure. Citrulline concentrations are currently used to reflect the residual intestinal mass in patients with SBS, although this method has several limitations. In a cohort of patients with SBS, we quantified apolipoprotein B-48 (ApoB-48), which is exclusively synthesized by enterocytes and secreted associated with dietary lipids and investigated the relationship between ApoB-48 and clinical and biological data as well as PN dependence. A total of 51 adult patients were included, 36 of whom were PN-dependent. We found a robust positive correlation between circulating ApoB-48 and residual small bowel length, which was also found in the subgroup of patients with jejunocolic anastomosis. Fasting ApoB-48 levels were significantly lower in PN-dependent patients than in PN-weaned patients and negatively correlated with parenteral nutrition dependence. Our results suggest that ApoB-48 could be proposed as a marker of intestinal absorptive function and could be an interesting follow-up marker in patients with SBS.

Citrulline correlations in short bowel syndrome-intestinal failure by patient stratification: Analysis of 24 weeks of teduglutide treatment from a randomized controlled study

BACKGROUND & AIMS

Disease-associated factors influence parenteral support (PS) reduction in response to teduglutide in patients with intestinal failure associated-short bowel syndrome (SBS-IF). We sought to determine correlative relationships between plasma citrulline levels, small bowel length, and PS volume.

METHODS

A post hoc analysis of plasma citrulline levels from patients in the STEPS 24-week study of teduglutide in patients with SBS-IF. Plasma citrulline was assessed in all patients; patients were stratified 3 times into subgroups based on bowel anatomy, cause of SBS-IF, and baseline PS volumes. Correlation analyses used simple linear regression models. Statistical comparisons between study groups were conducted using 2-sided t tests for 2 independent mean differences.

RESULTS

Baseline plasma citrulline correlated with remnant small bowel length (r = 0.355, P = 0.002), but not with baseline PS volume (r = -0.167, P = 0.14), in the overall population. There was a robust correlation between the baseline and Week 24 citrulline (r = 0.705, P < 0.0001), and an inverse correlation between change from baseline in citrulline and PS volume from baseline to Week 24 (r = -0.359, P = 0.001). In all subgroups, patients treated with teduglutide showed numerically greater increases in plasma citrulline at Week 24 compared with placebo.

CONCLUSION

Baseline plasma citrulline showed significant correlations with small bowel length in patients with ≥50% colon remaining/no stoma/colon-in-continuity, and patients with SBS-IF causes other than IBD/vascular disease. Citrulline levels may correlate with PS changes in response to teduglutide and more research may reveal a relationship between citrulline levels within the heterogeneous population of patients with SBS-IF. ClinicalTrials.gov NCT00798967, ClinicalTrialsRegister.eu 2008-006193-15.

Citrulline Generation Test: What Does It Measure?

BACKGROUND

The citrulline generation test (CGT) has been proposed as a tool to determine gut function. However, the increase in plasma citrulline concentration that follows a bolus dose of alanyl-glutamine may also result from a reduction in citrulline clearance due to competition with glutamine for transport.

MATERIALS AND METHODS

A swine model was developed, and stable isotope tracers were used to determine the mechanism behind the increase in plasma citrulline that follows a bolus dose of alanyl-glutamine. Plasma concentrations and enrichments were determined, and a non-steady-state model was used to calculate rates of appearance, disappearance, and conversion.

RESULTS

The pig model recapitulated the increase in plasma citrulline observed in humans after a dose of alanyl-glutamine. The dipeptide was rapidly hydrolyzed to its constitutive amino acids. Both citrulline plasma concentration and citrulline rate of appearance increased by ≈45% after the bolus dose of alanyl-glutamine. The conversion of citrulline to arginine and the rate of appearance of arginine also increased. Glutamine contributed up to 25% ± 2% of the rate of appearance of citrulline. No changes in the rate of disappearance of citrulline were observed.

CONCLUSION

Our results indicate that a single bolus dose of alanyl-glutamine increases plasma citrulline concentration by increasing citrulline production without any effect on citrulline disposal. Our findings strongly indicate that the CGT assesses the metabolic response of the gut and that CGT can become a useful tool to evaluate gut mass and function.

Citrulline as a marker of intestinal function and absorption in clinical settings: A systematic review and meta-analysis

Background

Citrulline has been described as a marker of intestinal function or absorption but evidence varies according to clinical settings.

Objective

The objective of this article is to examine the evidence of plasma citrulline as a marker of intestinal function and absorption in various clinical settings.

Methods

Studies were examined for p values, means and standard deviations, correlation coefficients or other metrics depicting the association of citrulline with intestinal function. A random effects model was used to produce a pooled estimate. A hierarchical summary receiver operating curve model was fitted for diagnostic accuracy measures.

Results

Citrulline levels are correlated strongly with small bowel length in short bowel syndrome patients (r = 0.67). Citrulline is strongly negatively correlated (r = -0.56) with intestinal disease severity with regards to enteropathies (coeliac disease, tropical enteropathy, Crohn's disease, mucositis, acute rejection in intestinal transplantation). Citrulline cut-off levels have an overall sensitivity and specificity of 80% and 84% respectively. Citrulline levels in untreated coeliac patients compared to controls were reduced by 10 µmol/l. Citrulline levels increase with gluten-free diet and with improvement of enteropathy. Citrulline is decreased in critical illness and sepsis.

Conclusion

These findings allow us to advocate quite reasonably that citrulline is a marker of acute and chronic intestinal insufficiency.

Intravenous citrulline generation test to assess intestinal function in intensive care unit patients

Background

Assessment of a quantifiable small intestinal function test is cumbersome. Fasting citrulline concentrations have been proposed as a measure of enterocyte function and elaborated into a citrulline generation test (CGT), which is applicable only when glutamine is administered orally. CGT is an oral test, limiting its use, for example, in critically ill patients.

Objective

Assessment of normative values and feasibility of an intravenously performed CGT in intensive care unit (ICU) patients with presumed gastrointestinal motility disturbances, especially when performed intravenously.

Design

CGT reference values were determined in 16 stable ICU patients using two different CGT methods, namely following either enteral or intravenous glutamine administration and both with simultaneous arterial and venous plasma citrulline sampling at six time-points. Plasma amino acid analysis was performed using reverse-phase high-performance liquid chromatography.

Results

The median total generation of citrulline in 90 min (CGT iAUCT90) was markedly higher with arterial citrulline sampling compared with venous citrulline sampling, being 724±585 and 556±418 µmol/L/min for enteral glutamine, respectively (p=0.02) and 977±283 and 769±231 µmol/L/min for intravenous glutamine, respectively (p=0.0004). The median slope (time-dependent increase) for plasma arterial and venous citrulline during the CGT was 0.20±0.16 and 0.18±0.12 µmol/L/min for enteral glutamine, respectively (p=0.004) and 0.22±0.16 and 0.19±0.05 µmol/L/min for intravenous glutamine, respectively (p=0.02).

Conclusion

Intravenous glutamine administration combined with arterial plasma citrulline sampling yielded the least variation in CGT characteristics in stable ICU patients. A 2-point measurement test had comparable test characteristics as a 6-point measurement CGT and seems promising.

Apo AIV and Citrulline Plasma Concentrations in Short Bowel Syndrome Patients: The Influence of Short Bowel Anatomy

INTRODUCTION

Parenteral nutrition (PN) dependence in short bowel syndrome (SBS) patients is linked to the functionality of the remnant small bowel (RSB). Patients may wean off PN following a period of intestinal adaptation that restores this functionality. Currently, plasma citrulline is the standard biomarker for monitoring intestinal functionality and adaptation. However, available studies reveal that the relationship the biomarker with the length and function of the RSB is arguable. Thus, having additional biomarkers would improve pointing out PN weaning.

AIM

By measuring concomitant changes in citrulline and the novel biomarker apolipoprotein AIV (Apo AIV), as well as taking into account the anatomy of the RSB, this exploratory study aims to a better understanding of the intestinal adaptation process and characterization of the SBS patients under PN.

METHODS

Thirty four adult SBS patients were selected and assigned to adapted (aSBS) and non-adapted (nSBS) groups after reconstructive surgeries. Remaining jejunum and ileum lengths were recorded. The aSBS patients were either on an oral diet (ORAL group), those with intestinal insufficiency, or on oral and home parenteral nutrition (HPN group), those with chronic intestinal failure. Apo AIV and citrulline were analyzed in plasma samples after overnight fasting. An exploratory ROC analysis using citrulline as gold standard was performed.

RESULTS

Biomarkers, Apo AIV and citrulline showed a significant correlation with RSBL in aSBS patients. In jejuno-ileocolic patients, only Apo AIV correlated with RSBL (rb = 0.54) and with ileum length (rb = 0.84). In patients without ileum neither biomarker showed any correlation with RSBL. ROC analysis indicated the Apo AIV cut-off value to be 4.6 mg /100 mL for differentiating between the aSBS HPN and ORAL groups.

CONCLUSIONS

Therefore, in addition to citrulline, Apo AIV can be set as a biomarker to monitor intestinal adaptation in SBS patients. As short bowel anatomy is shown to influence citrulline and Apo AIV plasma values, both biomarkers complement each other furnishing a new insight to manage PN dependence.

Effect of Teduglutide, a Glucagon-like Peptide 2 Analog, on Citrulline Levels in Patients With Short Bowel Syndrome in Two Phase III Randomized Trials

Objectives:

In clinical trials, treatment with the glucagon-like peptide 2 analog teduglutide was associated with improved fluid and nutrient absorption and increased intestinal villus height and crypt depth in patients with short bowel syndrome (SBS). Plasma citrulline, an amino acid produced by enterocytes, is considered a measure of enterocyte mass. This analysis assessed changes in plasma citrulline levels in patients with SBS in 2 phase III clinical studies of teduglutide.

Methods:

Both teduglutide studies (0.05 or 0.10 mg/kg/day in CL0600-004 and 0.05 mg/kg/day in CL0600-020) were phase III, 24-week, double-blind, and placebo controlled. Plasma citrulline levels were analyzed and validated by liquid chromatography coupled to tandem mass spectrometry.

Results:

In both the CL0600-004 and CL0600-020 studies, change in mean plasma citrulline concentrations at Week 24 vs. baseline was significantly greater with teduglutide compared with placebo (10.9 (0.05-mg/kg/day dose) and 15.7 (0.10-mg/kg/day dose) vs. 2.0 μmol/L and 20.6 vs. 0.7 μmol/L, respectively, for each study (P≤0.0001 for each comparison with placebo)). Teduglutide treatment was associated with reductions from baseline in PS (parenteral support) volume requirements; however, a significant correlation between PS reduction and increase in plasma citrulline at Week 24 was observed in only one out of the three teduglutide treatment groups.

Conclusions:

In 2 phase III studies, patients receiving teduglutide had significant increases in plasma citrulline at Week 24 compared with patients receiving placebo. Increases in plasma citrulline concentrations likely reflect enterocyte mass expansion, but no clear correlation was detected between change in plasma citrulline and change in weekly PS volume.

Serum citrulline as a diagnostic marker of sepsis-induced intestinal dysfunction

OBJECTIVE

To investigate the use of citrulline as an indicator for diagnosing septic acute intestinal dysfunction (SAID) in a rat model.

METHODS

SD rats were divided into three groups: a normal group (A), a model group (B), and a glutamine group (C). Group B was divided into a 36-h group (B1) and a 72-h feeding group (B2). The concentrations of serum citrulline, intestinal fatty acid-binding protein (I-FABP) and intestinal glutamine and histopathological changes were measured.

RESULTS

The lengths of the villus and thicknesses of the mucosal layer in groups B1, B2 and C were significantly different from those in group A. Citrulline concentrations in groups B1, B2 and C were lower than in group A; the serum concentrations in group C were significantly greater than in groups B1 and B2. The I-FABP levels of groups B1, B2 and C were higher than group A; I-FABP levels of groups B1 and B2 were higher than group C. Intestinal glutamine levels of groups B1 and B2 were lower than groups A and C. The serum citrulline of group C was negatively correlated with I-FABP and Chiu's score.

CONCLUSIONS

Serum citrulline could be used as the diagnostic indicator of SAID.

Serum markers in the clinical management of celiac disease

The advent of highly reliable noninvasive celiac diagnostic tests has transformed the field of celiac disease, from diagnosis, to evaluation of epidemiology, to clinical and translational research. Serologic tests in their modern forms are highly sensitive and specific for diagnosis, allowing for consideration of avoidance of diagnostic intestinal biopsy in some settings. On the other hand, as predictors of intestinal damage and for use in monitoring disease activity, currently available noninvasive tests have been disappointing. Serologic tests, while a measure of disease activity, do not correlate well with histology or symptomatology, and it is unclear if they predict long-term risk. Additionally, while the many clinically available tests have improved accessibility, they can have widely different cutoff levels and overall performance, making the comparison of levels in individual patients over time and across populations quite difficult. In the future, we can expect to see improvement in the currently available serologic tests including tissue transglutaminase and deamidated gliadin peptide with expansion of the dynamic range of the tests, and the celiac care community should push for a standardization of assays that would simplify research and patient care. Additionally, current serologic tests are measures of the adaptive immune response in celiac disease but do not directly measure intestinal inflammation. Promising work on intestinal fatty acid-binding protein and other assays which directly measure intestinal damage may complement traditional serologic tests and further improve our ability to noninvasively diagnose and monitor celiac disease. The coming years hold promise for the continuing evolution of serum-based tests in celiac disease with the possibility of substantial improvement of patient care and clinical research.

Citrulline levels following proximal versus distal small bowel resection

PURPOSE

Citrulline, a nonprotein amino acid synthesized by enterocytes, is a biomarker of bowel length and the capacity to wean from parenteral nutrition. However, the potentially variant effect of jejunal versus ileal excision on plasma citrulline concentration [CIT] has not been studied. This investigation compared serial serum [CIT] and mucosal adaptive potential after proximal versus distal small bowel resection.

METHODS

Enterally fed Sprague-Dawley rats underwent sham operation or 50% small bowel resection, either proximal (PR) or distal (DR). [CIT] was measured at operation and weekly for 8 weeks. At necropsy, histologic features reflecting bowel adaptation were evaluated.

RESULTS

By weeks 6-7, [CIT] in both resection groups significantly decreased from baseline (P<0.05) and was significantly lower than the concentration in sham animals (P<0.05). There was no difference in [CIT] between PR and DR at any point. Villus height and crypt density were higher in the PR than in the DR group (P≤0.02).

CONCLUSION

[CIT] effectively differentiates animals undergoing major bowel resection from those with preserved intestinal length. The region of intestinal resection was not a determinant of [CIT]. The remaining bowel in the PR group demonstrated greater adaptive potential histologically. [CIT] is a robust biomarker for intestinal length, irrespective of location of small intestine lost.

Intestinal adaptation following resection

Intestinal adaptation is a natural compensatory process that occurs following extensive intestinal resection, whereby structural and functional changes in the intestine improve nutrient and fluid absorption in the remnant bowel. In animal studies, postresection structural adaptations include bowel lengthening and thickening and increases in villus height and crypt depth. Functional changes include increased nutrient transporter expression, accelerated crypt cell differentiation, and slowed transit time. In adult humans, data regarding adaptive changes are sparse, and the mechanisms underlying intestinal adaptation remain to be fully elucidated. Several factors influence the degree of intestinal adaptation that occurs post resection, including site and extent of resection, luminal stimulation with enteral nutrients, and intestinotrophic factors. Two intestinotrophic growth factors, the glucagon-like peptide 2 analog teduglutide and recombinant growth hormone (somatropin), are now approved for clinical use in patients with short bowel syndrome (SBS). Both agents enhance fluid absorption and decrease requirements for parenteral nutrition (PN) and/or intravenous fluid. Intestinal adaptation has been thought to be limited to the first 1-2 years following resection in humans. However, recent data suggest that a significant proportion of adult patients with SBS can achieve enteral autonomy, even after many years of PN dependence, particularly with trophic stimulation.

Diagnosing small bowel malabsorption: a review

Malabsorption encompasses dysfunctions occurring during the digestion and absorption of nutrients. A small proportion of patients presents with chronic diarrhoea. A clinical history supportive of malabsorption may guide investigations toward either the small bowel or pancreas. Serological testing for coeliac disease will determine most cases without invasive investigations. In the clinical context of persisting weight loss and malnutrition, small bowel enteropathy may be investigated with small intestinal biopsies. Small bowel absorptive capacity and permeability might be measured by oral sugar-mix ingestion. Further, approaches to the investigation of malabsorption might also involve the detection in faeces of a substance that has not been absorbed. A variation of the latter is the use of breath testing which relies on the breakdown of the malabsorbed test substance by colonic flora. Measurement of protein absorption is difficult and unreliable; it is, therefore, rarely advocated in clinical settings. No single biological marker confirming a diagnosis of small bowel malabsorption or small bowel integrity is presently available in clinical practice. Plasma citrulline concentration, an amino acid not incorporated into endogenous or exogenous proteins, has been extensively used in research studies and supportive results are establishing its concentration as a reliable quantitative biomarker of enterocyte absorptive capacity.

Teduglutide, a novel glucagon-like peptide 2 analog, in the treatment of patients with short bowel syndrome

Short bowel syndrome results from surgical resection, congenital defect or disease-associated loss of absorption. Parenteral support (PS) is lifesaving in patients with short bowel syndrome and intestinal failure who are unable to compensate for their malabsorption by metabolic or pharmacologic adaptation. Together, the symptoms of short bowel syndrome and the inconvenience and complications in relation to PS (e.g. catheter-related blood steam infections, central thrombosis and intestinal failure associated liver disease) may impair the quality of life of patients. The aim of treatment is to maximize intestinal absorption, minimize the inconvenience of diarrhea, and avoid, reduce or eliminate the need for PS to achieve the best possible quality of life for the patient. Conventional treatments include dietary manipulations, oral rehydration solutions, and antidiarrheal and antisecretory treatments. However, the evidence base for these interventions is limited and treatments that improve the structural and functional integrity of the remaining intestine are needed. Teduglutide, an analog of glucagon-like peptide 2, improves intestinal rehabilitation by promoting mucosal growth and possibly by restoring gastric emptying and secretion, thereby reducing intestinal losses and promoting intestinal absorption. In a 3-week, phase II balance study, teduglutide reduced diarrhea by around 700 g/day and fecal energy losses by around 0.8 MJ/day. In two randomized, placebo-controlled, 24-week, phase III studies, similar findings were obtained when evaluating the fluid composite effect, which is the sum of the beneficial effects of teduglutide - reduction in the need for PS, increase in urine production and reduction in oral fluid intake. The fluid composite effect reflects the increase in intestinal fluid absorption (and the concomitant reduction in diarrhea) and may be used in studies in which metabolic balance assessments are not performed. In studies of up to 24 weeks' duration, teduglutide appears to be safe and well tolerated. Treatment with teduglutide was associated with enhancement or restoration of the structural and functional integrity of the remaining intestine with significant intestinotrophic and proabsorptive effects, facilitating a reduction in diarrhea and an equivalent reduction in the need for PS in patients with short bowel syndrome and intestinal failure.

Plasma citrulline concentration as a biomarker for bowel loss and adaptation in hospitalized pediatric patients requiring parenteral nutrition

BACKGROUND

Citrulline is a nonessential amino acid produced solely in the enterocyte. Plasma citrulline concentration has been proposed as a noninvasive biomarker for bowel length, function, and dependency on parenteral nutrition (PN). The purpose of this study was to determine if citrulline concentrations differed between pediatric patients with and without small bowel loss requiring specialized nutrition support.

METHODS

This was a retrospective categorical analysis of citrulline concentrations from previously published studies. Patients were included if they were receiving PN, more than 30 days of age, and if they had at least 2 plasma citrulline concentrations. Patients with renal insufficiency and who received outpatient PN treatment were excluded. Patients were categorized as either having or not having small bowel loss.

RESULTS

Thirty-six patients were included for analysis (18 per category). The median citrulline concentration was significantly lower in the group with bowel loss, 8.4 µmol/L vs 10.5 µmol/L (P < .0005), and undetectable citrulline concentrations occurred more often in the bowel loss group, 40% vs 8% (P < .0005). In 13 patients who received enteral nutrition during the study periods, plasma citrulline concentrations increased only in patients without bowel loss.

CONCLUSIONS

These data confirm previous studies and identify decreased citrulline concentrations in pediatric patients with bowel dysfunction in the absence of bowel loss. These data also represent the first serial citrulline concentrations over a 21-day period. The increase in citrulline concentrations only in fed patients without bowel loss suggests that citrulline concentrations could provide a biomarker for bowel function and adaptation.

Assessment of Small Bowel Function in Critical Illness: Potential Role of Citrulline Metabolism

Small intestinal function in critically ill patients should ideally be assessed in order to determine the preferred feeding route, timing, and composition of enteral nutrition. Additionally, evaluation of small bowel function may lead to new insights aimed to maintain enterocyte integrity. Critically ill patients are likely to have impaired enterocyte function mainly as a consequence of diminished splanchnic blood flow associated with mucosal hyperpermeability and bacterial translocation, a pathological state believed to be pivotal in the development of sepsis and multiple organ dysfunction syndrome (MODS). However, feasible and validated clinical tools to reliably assess enterocyte function are lacking. This explorative review discusses the promising role of citrulline, a nonprotein amino acid almost exclusively generated by the enterocyte, as a biomarker reflecting enterocyte function in critically ill patients. Citrulline metabolism, its potential as enterocyte biomarker, and literature on citrulline in critically illness will be discussed. Finally, a novel test for enterocyte function, the citrulline generation test (enterocytic citrulline production upon stimulation with enteral or intravenous glutamine) will be considered briefly.

Serum markers and intestinal mucosal injury in chronic gastrointestinal ischemia

BACKGROUND

Diagnosing chronic gastrointestinal ischemia (CGI) is a challenging problem in clinical practice. Serum markers for CGI would be of great diagnostic value as a non-invasive test method.

AIMS

This study investigated serum markers in patients with well-defined ischemia. Furthermore, intestinal mucosal injury was also evaluated in CGI patients.

METHODS

Consecutive patients suspected of CGI were prospectively enrolled and underwent a diagnostic work-up consisting of gastrointestinal tonometry and either CT or MR angiography. Blood samples for analysis of intestinal fatty acid-binding protein (I-FABP), D-dimer, lactate dehydrogenase (LDH), leucocyte counts, C-reactive protein (CRP), and L-lactate were drawn before and after a standard meal. Intestinal mucosal injury was assessed with glutamine, citrulline and arginine in blood samples and compared to a sugar absorption test (SAT). Test reproducibility was validated in healthy subjects.

RESULTS

Forty patients and nine healthy subjects were included. Ischemia was diagnosed in 32 patients (80%). I-FABP, leucocyte counts, LDH, CRP, glutamine, citrulline, arginine and SAT levels did not differ between patients with and without ischemia. L-lactate concentration showed a significant elevation in ischemia patients as compared to non-ischemia patients. In ischemia patients, D-dimer levels showed a significant elevation postprandially as compared to D-dimer levels at baseline. However, these ischemia patients did not show intestinal mucosal injury.

CONCLUSIONS

I-FABP, leucocyte counts, LDH and CRP levels are not clinically useful for the diagnosis of CGI. However, postprandial rises in L-lactate and D-dimer serum levels can serve as non-invasive indicators of CGI.

Plasma citrulline kinetics and prognostic value in critically ill patients

PURPOSE

Multiple organ failure is a leading cause of death in critically ill patients and could be secondary to early gut ischemia. Plasma citrulline is a biomarker of enterocyte mass, and critically ill patients may have enterocyte mass reduction. The objectives of this study were to assess plasma citrulline kinetics and its prognostic value in critically ill patients.

METHODS

This prospective observational study included adults without small bowel disease and without chronic renal failure consecutively admitted to a single intensive care unit. Prognostic variables as well as plasma citrulline concentrations were studied at admission, 12, 24, 48 h, and the 7th day after admission. Univariate and multivariate analyses including plasma citrulline (0-10, 11-20, and >20 micromol l(-1)) and other variables were performed.

RESULTS

Sixty-seven patients were included, and the 28-day mortality was 34%. During the 1st day mean plasma citrulline decreased from 18.8 to 13.5 micromol l(-1). Low plasma citrulline at 24 h was associated with low plasma glutamine and arginine (p = 0.01 and 0.04), and high plasma CRP concentration, nosocomial infection rate, and 28-day mortality (p = 0.008, 0.03, and 0.02, respectively). In multivariate analysis plasma citrulline < or = 10 micromol l(-1) at 24 h and SOFA score > or =8 at 24 h were associated with 28-day mortality(odds ratios 8.70 and 15.08).

CONCLUSIONS

In critically ill patients, low plasma citrulline at 24 h is an independent factor of mortality and could be a marker of acute intestinal failure.

Diagnosing malabsorption in the intensive care unit

Malabsorption as a result of decreased intestinal function is a frequently occurring problem in intensive care units. Small bowel dysfunction may lead to malnutrition and may predispose patients to infectious complications (sepsis) and may be linked to increased hospitalization duration, morbidity and mortality. There are several small bowel function tests, such as faecal fat excretion and sugar absorption tests, but data specifically applicable to the intensive care setting are limited. Moreover, there are multiple confounders such as altered intestinal permeability, changed transit time and renal dysfunction. A more specific test, fasting plasma citrulline concentration, has poor test characteristics for detection of decreased intestinal function in patients with dysfunctional enterocytes. Therefore, a citrulline generation test was recently developed in order to measure the conversion of glutamine (administered either enterally or intravenously) into citrulline. This conversion takes place exclusively in a properly functioning enterocyte; therefore, citrulline generation upon glutamine stimulation may be a promising biomarker for assessment of the small bowel function.

Plasma citrulline is a biomarker of enterocyte mass and an indicator of parenteral nutrition in HIV-infected patients

BACKGROUND

Plasma citrulline is a biomarker of enterocyte mass and function in humans.

OBJECTIVE

We evaluated citrulline in the reemerging context of diarrhea in HIV-infected patients receiving highly active antiretroviral therapy.

DESIGN

This study prospectively measured citrulline in 6 groups of HIV-1 patients (n = 115): 1) undetectable viral load without chronic diarrhea (a; n = 40) and with protease inhibitor-associated toxic chronic diarrhea (b; n = 26), 2) detectable viral load and CD4 > 200/mm(3) without (a; n = 6) and with (b; n = 11) chronic diarrhea, and 3) detectable viral load and CD4 <200/mm(3) without chronic diarrhea (a; n = 7) and with opportunistic intestinal infections or HIV enteropathy (b; n = 25). The influence of diarrhea on citrulline was assessed by comparing the a and b subgroups with healthy control subjects (n = 100).

RESULTS

Citrulline was slightly decreased (22-30 micromol/L) in groups 1b and 2b and was <22 micromol/L in 19 of 25 patients in group 3b. In group 3b, a citrulline concentration <10 micromol/L was associated with a clinical indication for parenteral nutrition (n = 6 of 8 compared with 2 of 17 if the citrulline concentration was >10 micromol/L; P < 0.05). Citrulline correlated positively with albumin (P < 0.01) and BMI (P < 0.05) and negatively with C-reactive protein (P < 0.01). When antiinfectious and nutritional therapies were successful (n = 18 of 25), citrulline normalized in 2-12 wk. Neither chronic hepatic or pancreatic disease nor lipodystrophy and the metabolic syndrome affected citrulline. Compared with control subjects (38 +/- 8 micromol/L), patients without chronic diarrhea (groups 1a, 2a, and 3a) had normal citrulline concentrations (36 +/- 6 micromol/L).

CONCLUSIONS

Plasma citrulline is a reliable biomarker of enterocyte functional mass in HIV patients. Citrulline does not allow the etiologic diagnosis of enteropathy, but it can discriminate between protease inhibitor toxic diarrhea and infectious enteropathy and quantify the functional consequences, which makes it an objective tool for indicating the need for parenteral nutrition.

Is serum citrulline measurement clinically useful in coeliac disease?

Citrulline (CIT), a non-protein amino acid in circulating blood, is almost exclusively contained in the enterocytes of small bowel mucosa and may represent a reliable marker of functioning enterocyte mass. The aim of this study was to evaluate the clinical utility of measuring serum citrulline levels in a group of patients affected by coeliac disease (CD). Fifty healthy volunteers, 21 patients with untreated coeliac disease and 6 patients with refractory coeliac disease took part in the study. Serum citrulline levels and duodenal lesions were evaluated at the time of diagnosis, and after at least 24 months of gluten-free diet. Serum citrulline concentrations were determined by ion exchange chromatography. In comparison to healthy volunteers, serum citrulline concentrations were significantly lower in untreated and refractory coeliac disease patients. No significant difference was found between untreated and refractory coeliac disease patients and between patients with different patterns of clinical presentation or various degrees of duodenal lesions. After a gluten-free diet, the mean of serum citrulline concentration was increased in all but one patient. Although, as expected, serum citrulline levels turned out to be low in coeliac disease, the clinical utility of their measurement is, at least, questionable in this condition.

The citrulline generation test: proposal for a new enterocyte function test

BACKGROUND

The amino acid citrulline is mainly produced by enterocytes from conversion of glutamine. As fasting plasma citrulline proved disappointing as a biomarker for enterocyte dysfunction in clinical practice, we propose a citrulline generation test (CGT) to assess enterocyte function.

AIM

To assess the feasibility of a CGT in healthy subjects and patients with decreased enterocyte mass.

METHODS

Nineteen healthy subjects, 16 patients with intestinal villous atrophy and nine patients with short bowel syndrome (SBS) were given an oral bolus of 20 g of the dipeptide alanine-glutamine. Subsequent changes in plasma citrulline and other amino acid concentrations were determined using reverse-phase high-performance liquid chromatography.

RESULTS

Following the oral bolus of alanine-glutamine, plasma citrulline concentrations showed a time dependent rise in healthy subjects of 44 +/- 13% (38-55 micromol/L, P < 0.0001). The slope from baseline plasma citrulline to peak concentrations was 0.22 +/- 0.08, 0.13 +/- 0.04 and 0.09 +/- 0.04 micromol/L/min in healthy subjects, patients with coeliac disease (CeD) and refractory CeD, respectively (healthy subjects vs. CeD P < 0.05, healthy subjects vs. refractory CeD P < 0.001). In patients with SBS, the CGT was able to distinguish between non-adapted and adapted SBS by means of the incremental area under the CGT curve till 90 min (iAUC T90). The iAUC T90 was 447 +/- 179 and 1039 +/- 178 micromol/L/min in non-adapted and adapted SBS, respectively (P = 0.04).

CONCLUSION

An oral bolus of alanine-glutamine induces a time-dependent rise in plasma citrulline concentration to an extent dependent on the existence of villous atrophy or enterocyte hyperplasia in CeD, and adapted SBS, respectively.

Citrulline as a biomarker of intestinal failure due to enterocyte mass reduction

BACKGROUND & AIMS

In human, citrulline (plasma concentration about 40 micromol/L) is an amino acid involved in intermediary metabolism and that is not incorporated in proteins. Circulating citrulline is mainly produced by enterocytes of the small bowel. For this reason plasma or serum citrulline concentration has been proposed as a biomarker of remnant small bowel mass and function. This article reviews this concept and its metabolic basis.

METHODS

Conditions in which there is a significantly reduced small bowel enterocyte mass and function and a plasma or serum citrulline were measured in adults and children. These studies included patients with a short bowel syndrome, villous atrophy states, Crohn's disease, during monitoring of digestive toxicity of chemotherapy and radiotherapy or follow-up of patients after small bowel transplantation.

RESULTS

In all these situations, with more than 500 studied patients a decreased level of plasma citrulline correlated with the reduced enterocyte mass independently of nutritional and inflammatory status. A close correlation between small bowel remnant length and citrullinemia was found. In addition, diagnosis of intestinal failure was assessed through plasma citrulline levels in severe small bowel diseases in which there is a marked enterocyte mass reduction.

DISCUSSION

The threshold for establishing a diagnosis of intestinal failure is lower in villous atrophy disease (10mumol/L) than in short bowel syndrome (20mumol/L). Compromised renal function is an important factor when considering plasma citrulline levels as a marker of intestinal failure as this potentially can increase circulating citrulline values.

CONCLUSIONS

Reduced plasma citrulline levels are an innovative quantitative biomarker of significantly reduced enterocyte mass and function in different disease states in humans.