The diagnostic methods of hepatic fibrosis include LB, imaging methods, and serum marker. LB is the current gold standard for the diagnosis of hepatic fibrosis [12
]. However, LB also has a few limitations. First, the biopsy procedure results in pain in 24.6% of patients and poses risk of severe complications in 0.31% of patients [13
]. Second, it has been shown that there is a high inter- and intra-observer variation among pathologists in determining the stage of liver fibrosis using biopsy specimens [5
]. Third, histological staging is based on a biopsy specimen that represents at most 1/50,000 of the total liver mass [14
], and the distribution of fibrosis in the liver parenchyma is heterogeneous, which results in a non-negligible sampling error. Siddique et al.
, found that different LB specimens (at least 15 mm long) taken at the same puncture site often indicate different liver fibrosis stages in 45% of studied patients [8
]. Fourth, LB is not well accepted by patients. A French survey recently showed that approximately half of patients with hepatitis C virus infection refuse to be referred to hepatologists for fear of LB [4
]. Imaging methods for diagnosing hepatic fibrosis include ultrasound, CT, MRI and transient elastography (FibroScan). It has been demonstrated that ultrasound, CT, and MRI are inadequate to diagnose and differentiate early stages of fibrosis, and the diagnosis of cirrhosis is often based on signs of advanced liver cirrhosis. FibroScan is a novel, noninvasive, and rapid bedside method for assessing liver fibrosis by measuring liver stiffness. Although recent data supports the diagnostic value of transient elastography for hepatic fibrosis, it can only diagnose liver cirrhosis and severe fibrosis, and the diagnostic accuracy for early stages of fibrosis (Metavir score F1 and F2) is poor [15
]. In addition, FibroScan may not give accurate diagnosis for hepatic fibrosis in obese patients and cannot detect cirrhosis in patients with acute liver damage. Moreover, since liver stiffness is likely to change with age, the patient’s age affects the accuracy of FibroScan as well [16
]. Serum marker of liver fibrosis is closely linked to the pathophysiological abnormality of fibrogenesis. Hepatic fibrosis is characterized by an excessive deposition of extracellular matrix (ECM) proteins caused by both increased synthesis and decreased or unbalanced degradation of ECM [17
], which lead to higher levels of circulating ECM components or their fragments in the peripheral blood. Serum markers identified as having diagnostic value for hepatic fibrosis mainly include hyaluronic acid (HA), laminin, fibronect, procollagens I and III, and type IV collagens. Among these markers, HA has been considered to have good accuracy for the diagnosis of hepatic fibrosis. Other serum markers related to the mechanism of hepatic fibrosis such as matrix metalloproteinases (MMP), tissue inhibitors of metalloproteinases (TIM), transforming growth factor β (TGF), and tumor necrosis factor (TNF) β may have low sensitivity and specificity. The ideal marker should have the following characteristics: (1) specific and sensitive for liver fibrosis; (2) accurate staging of hepatic fibrosis; (3) unaffected by comorbidities; (4) reproducible; (5) non-invasive; and (6) cost effective. So far, serum markers that possess all the advantages described above for the diagnosis of hepatic fibrosis have not been identified.
Many serum proteins and non-protein molecules without exogenous fluorescent substances can produce AF after UV (ultra-violet) excitation at the appropriate wavelength. AF detection has three major advantages over other light-based investigation methods: highly sensitive, quick, and safe. AF detection has been used recently in the field of cancer diagnosis. Masilamani et al.
] found that blood components of patients with gastric cancer, breast cancer, and Hodgkin’s lymphoma showed distinct and enhanced fluorescence intensity (around 630 nm) due to the porphyrin fluorophore. However, serum AF has never been indicated to be applicable in hepatic fibrosis diagnosis. Here we demonstrate significantly higher serum AF intensity in liver fibrotic rats compared with normal rats, and there is a significant positive correlation between serum AF intensity and hepatic fibrosis stages. Many experts consider non-invasive tests for fibrosis with an AUROC value of 0.85–0.90 to be as good as liver biopsies for staging fibrosis [19
]. The AUROC values for serum AF tests for F
≥ 2 (0.876), F
≥ 3 (0.994), and F
= 4 (0.997) are higher than 0.85, suggesting that serum AF is a good marker for staging fibrosis.
Serum AF detection for diagnosis of hepatic fibrosis has a number of advantages. First of all, it is non-invasive when compared to LB. Secondly, it is simple, rapid, and economical when compared to traditional serum markers, such as HA detection. For HA detection, Antibody kits, and antigen-antibody reaction time are required. Therefore, serum AF detection may potentially become a better diagnostic method for hepatic fibrosis, especially severe hepatic fibrosis.
So far it is unclear what substance in hepatic fibrosis rat serum generates autofluorescence at 512 nm when excited at 377 nm. In this study, rat liver fibrosis was induced by carbon tetrachloride that can induce lipid peroxidation products. Lipid peroxidation products play an important role in the pathogenesis of liver fibrosis. Lipid peroxidation products have a maximum excitation wavelength at 360 nm and a maximum emission wavelength at 440 nm [20
]. Advanced glycation end products have the characteristics of autofluorescence, and they increase in serum of patients with cirrhosis, but they generate more intense fluorescence at 440 nm when excited at 370 nm [21
]. The excitation peak and emission peak of serum bilirubin are 460 nm and 515 nm, respectively. Collagen type I and type V have AF of emission maxima at 430 and 480 nm after excitation at 332 nm [23
]. All data above suggest that the substance producing autofluorescence (excitation wavelength 337 nm, emission wavelength 512 nm) may not be lipid peroxidation products, advanced glycation end products, bilirubin, or collagen type I and type V. It is speculated that other ECM components may be the substance of serum autofluorescence in rats with liver fibrosis. Further investigation is needed to elucidate the source of serum autofluoresecnce.