Acute phase reactants
An extensive list of acute phase reactants is provided in Table 2 but those relevant to IBD are discussed.1 Haemoglobin, platelet count, mean platelet volume, erythrocyte sedimentation rate (ESR), serum thrombopoietin, serum erythropoietin, C-reactive protein and orosomucoid (α1-acid glycoprotein) have been used for assessment of disease activity singly or in combination. ESR and C-reactive proteins are the most commonly used markers.
Table 2. Acute phase reactants
|Protein whose plasma concentrations increase|| |
| Complement system||C3, C4, C9, factor B, C1 inhibitor, C4b-binding protein, mannose-binding lactin|
| Coagulation and fibrinolytic system||Fibrinogen, plasminogen, tissue plasminogen activator, urokinase, protein S, vitronectin, plasminogen activator inhibitor 1|
| Antiproteases||α1-Protease inhibitor, α1-antichymotrypsin, pancreatic secretory trypsin inhibitors, inter-α-trypsin inhibitors|
| Transport proteins||Ceruloplasmin, haptoglobulin, hemopexin|
| Participants in inflammatory responses||Secreted phospholipase A2, lipopolysaccharide-binding protein, interleukin-1 receptor antagonists, granulocyte colony-stimulating factor|
| Others||C-reactive protein, serum amyloid A, α1-acid glycoprotein, fibronectin, ferritin, angiotensinogen|
|Proteins whose plasma concentrations decrease||Albumin, transferrin, transthyretin, α2-HS glycoprotein, α-fetoprotein, thyroxine-binding globulin, insulin-like growth factor 1, factor XII|
Erythrocyte sedimentation rate
Erythrocyte sedimentation rate is an indirect measurement of plasma acute phase protein concentration and is influenced by erythrocytes morphology as well as plasma constituents such as immunoglobulins.1 As the concentrations of many serum proteins vary in patients with IBD and as some have long half lives, the ESR is not rapidly responsive to change in clinical status (the ESR may take several days to decrease even when rapid clinical improvement occurs). Hence, the ESR is a crude assessment of disease activity. In ulcerative colitis (UC), where clinical, endoscopic and histological activity is used to assess the overall disease, the correlation between ESR and disease activity is good.1–5 However, it may be normal in proctitis and proctosigmoiditis. In Crohn's disease (CD), the ESR appears to be a less accurate measure of disease activity. The ESR does appear to increase with increasing disease activity but this correlates more with colonic disease and does not reflect the disease activity of small bowel.2–5
C-reactive protein is produced as an acute phase reactant predominantly in the liver in response to stimulation by interleukin (IL)-6, TNF-α and IL-1β, which are produced at the site of inflammation.8 CRP functions as an opsonin for bacterial sequences and nuclear material expressed during apoptosis. CRP has been used as a marker to diagnose and to predict the activity of inflammatory disease. CRP synthesis by the liver is the only factor determining plasma CRP concentration. Hence, only the liver failure or therapies affecting acute phase stimulus may decrease CRP.8 The normal value of CRP for systemic inflammation is 0.8 mg/dL (8 mg/L). In the presence of acute phase stimulus, CRP production is rapidly upregulated. Once the acute phase stimulus disappears, CRP concentrations quickly decrease due to short half-life of 19 h. This makes CRP a valuable marker to detect the activity of IBD. In CD, serum levels of CRP correlate with disease activity. For UC, same trend can be observed although CRP is overall lower than in CD. In a recent study, CRP elevation was significantly associated with moderate to severe clinical activity, active disease at ileocolonoscopy and histological severe inflammation (Table 3) but not with abnormal small bowel imaging in patients with CD. In UC patients, CRP elevation was significantly associated with severe clinical activity and active disease at colonoscopy but not with histological inflammation.9 There is a good correlation between CRP and other measures of inflammation such as the CD activity index, radioactive-labelled faecal granulocyte excretion and faecal calprotectin.3, 10, 11 However, some patients have low CRP in the face of active disease. These patients often have exclusive ileal disease and low body mass index.12 The value of CRP as a predictor of relapse is controversial with some studies finding it an accurate predictor and others not.13–15
Table 3. Clinical disease activity and laboratory parameters of inflammation (CRP, ESR) in 147 IBD patients
| ||CD tested, N (%)||UC tested, N (%)|
|Clinically active CD/ UC||91/104||38/43|
|CRP elevated||47/104 (45)||19/43 (44)|
|ESR elevated||16/51 (31)||19/43 (44)|
Leucocytosis is not a useful marker of disease activity in clinical practice as there are many factors besides disease activity (systemic glucocorticosteroids, immunosuppressants, presence of abscess) that affect it. Platelet count correlates with disease activity in IBD but it is not used in clinical practice in IBD as there are other factors such as haemorrhage from other sites and iron deficiency anaemia which can cause elevation of platelet count.16, 17 Neopterin, an intermediate metabolite in the synthetic pathway of biopterin, is synthesized and released from monocytes/macrophages upon non-specific stimulation. The level of neopterin in urine and serum has been shown to correlate with disease activity of UC and CD but this is not IBD specific.18, 19
Leucocytes do not readily adhere to vascular endothelium in an unstimulated state. However, inflammatory signals induce the expression of proteins on the endothelial cell surface that promote the adhesion and extravasation of activated immune cells from the circulation into the underlying tissues.40 These proteins are known as cell adhesion molecules and they are expressed by immune cell, endothelial cell and epithelial cells. They can be in soluble form or leucocyte-bound form. Amongst these key molecules are L-, P- and E-selectin and cell adhesion molecules [mucosal adressin-cell adhesion molecule (MAdCAM-1), intercellular adhesion molecule-1 (sICAM-1) and vascular cell adhesion molecule-1 (sVCAM-1) on the endothelial cells]. Integrins are secondary adhesion molecules and need to be activated before they engage in cell adhesion. The integrins interact with specific ligands, the addressins on the endothelium (α4β1-integrin binds to VCAM-1 and α4β7-integrin binds to MAdCAM-1).41
The serum concentrations as well as their in situ expression have been studied in IBD. sICAM-1 and sE-selectin are elevated in the serum of patients with IBD but CRP and microalbuminuria reflect clinical disease activity more accurately.42 There is significant overlap between UC and CD, between IBD and controls and between inactive and active IBD. Thus, there is no support to the routine use of soluble adhesion molecules as disease activity markers in IBD.
Lactoferrin is a glycoprotein found in many body fluids as well as in granules of neutrophil granulocytes. Faecal lactoferrin levels quickly increase after influx of neutrophils into intestinal lumen during inflammation. Faecal lactoferrin is measured by ELISA on a single stool sample. The concentration faecal lactoferrin in healthy individuals is 1.45 ± 0.4 μg/g of faecal weight. In active IBD this value can rise to several hundred.55 Faecal lactoferrin concentration is increased in patients with active IBD when compared to those with inactive IBD with specificity between 85% and 90%. Faecal lactoferrin levels may rise significantly prior to a clinically evident relapse and may be a good marker to predict subsequent IBD flares.56, 57
Recent studies comparing faecal lactoferrin and calprotectin have suggested that both tests are similarly useful in the assessment of the disease activity of IBD. Faecal excretion of calprotectin correlated with finding of colonic inflammation at endoscopy while faecal excretion of lactoferrin correlated with histological inflammation.58, 59
Prediction of relapse in inflammatory bowel disease
If relapse can be predicted in IBD, it is likely to change the approach to treatment. Laboratory markers have been studied to predict a relapse, more in CD than UC.
Brignola et al. formulated a prognostic index based on ESR, α2-globulin and α1-glycoprotein (Table 4).61 High value (prognostic index > 0.35) predicted relapse at 18 months. Normal values did not predict remission in all patients.
Table 4. A laboratory index for predicting relapse in asymptomatic patients with Crohn's disease61
|−3.5 + (ESR × 0.03) + (acid α1-glycoprotein × 0.013) + (α2-globulin × 2)|
As mentioned earlier, CRP value has been studied to predict relapse with some studies finding it useful while others have not. In a study of patients with CD, those with clinically inactive disease with elevated CRP had higher chance of relapse in the following 2 years than those with normal CRP.13 A combination of CRP and ESR has also been used to predict relapse in patients with CD.14 Patients with CRP > 20 mg/L and an ESR > 15 mm/first hour had an eightfold increased risk of relapse in the next 6 weeks.
Reinisch et al. found that serum levels of IL-6 were useful to predict relapse in steroid-induced remission in patients with CD.62 Louis et al. found that high serum level of soluble IL-2 receptors in patients with CD was highly predictive of relapse and was complementary to other inflammatory and clinical markers in the prediction of relapse of disease.63
The role of faecal calprotectin and lactoferrin in prediction of relapse of IBD has been discussed earlier. The level of faecal calprotectin in prediction of relapse needs further study.
Bitton et al. followed ESR, CRP, IL-1β, IL-6 and IL-15 values at base line, 6 months and 12 months and found that they were not useful in predicting a relapse in UC.64 Yamomoto et al. studied conventional blood markers, plasma and mucosal levels of cytokines (IL-1β, IL-6, TNF-α and IL-8) to predict relapse in patients with UC.65 Only higher rectal mucosal IL-8 levels were significantly associated with relapse. Thus, faecal markers and rectal mucosal IL-8 are the only promising biomarker for prediction of relapse in patients with UC.
Small intestinal permeability has been recently studied to predict the relapse in patients with small intestinal CD and had sensitivity and specificity of 84% and 61%, respectively, to predict the relapse.46 Thus, some of the biomarkers may be useful in predicting a relapse.
In conclusion, assessment of disease activity in patients with IBD is important both in clinical practice and in clinical trials. Biological disease activity markers have been studied for assessment of disease activity and to predict relapse in patients with IBD. These include serological markers such as acute phase reactants, cytokines and adhesion molecules, faecal markers such as calprotectin and lactoferrin, measurement of intestinal permeability, white cell scan and measurement of gut lavage fluid protein. C-reactive protein was found to be useful in some studies. The data on ILs, IL-Rs and IL-RAs and adhesion molecules are limited or inconsistent. Faecal calprotectin and lactoferrin are promising markers. For predicting relapse, CRP, ESR, IL-2, IL-6, IL-8 and faecal markers may be of value.