Inclusion and exclusion criteria
Eligible studies were those original research articles that directly compared the diagnostic accuracy of GP73 test with AFP for HCC in the same patients, or randomly assigned patients to one of the tests using blood as the only sample type. Studies that evaluated serum GP73 or AFP levels by messenger RNA, DNA or DNA polymorphisms, or those without providing the sensitivity or specificity of GP73 or AFP were excluded.
Studies published in English and Chinese were included. Abstracts, letters, editorials and expert opinions, reviews without original data, case reports and studies lacking control groups were also excluded. No restriction was set on the year of publication.
Identification of studies
A comprehensive systematic literature review of original researches studying the diagnostic accuracy of GP73 was performed searching the following electronic databases through May 2011: PUBMED, EMBASE, Chinese BioMedical Literature Database (CBM), Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, Database of Abstracts of Reviews of Effect (DARE), Health Technology Assessment Database and NHS Economic Evaluation Database (NHS-EED). In addition, references from included articles and relevant published reports were hand searched. No restriction was set on the language, study design, year of publication and publishes status. Subject headings and keywords used in the search process included the following: (1) GP73: GP73, golgi protein 73, golgi phosphoprotein 2, golgi membrane protein 1; and (2) HCC: HCC, hepatocellular carcinoma, liver cell carcinoma, hepatic cell carcinoma. The PUBMED search strategy was shown in Additional file 1
. We did not use keywords or indexing terms for diagnostic test accuracy since they might miss relevant studies.
All the studies were reviewed by two reviewers (Zhou Y and Ying J) independently based on titles and abstracts, and then the full texts of potentially eligible studies were retrieved for further assessment. Disagreements between the reviewers were resolved by consensus. The authors would be contacted for further study details when necessary.
When the same author reported results obtained from the same patient population in several publications, only the most recent or the most complete report was included in the analysis to avoid overlap between cohorts.
The following data were extracted from the included studies by two reviewers (Zhou Y and Yin X) independently: authors, year of publication, journal, study design, number of patients, reference test, assay type of the markers, cutoff values and raw data for the analysis of sensitivity and specificity (the number of true positive, false negative, true negative and false positive results) for comparison of patients diagnosed with HCC vs. control. Any disagreements were resolved through consultation with the third reviewer (Zhang BH).
Assessment of methodological quality
The quality of each study was assessed according to the QUADAS (Quality Assessment of studies of Diagnostic Accuracy included in Systematic reviews) checklist recommended by Cochrane Collaboration. Each of the 11 items in the QUADAS checklist was scored as "yes", "no", or "unclear" [8
Representative patient spectrum
HCC typically develops in patients with chronic liver disease and cirrhosis [2
]. It is in these target populations that serum markers are most urgently needed. Patients with chronic liver disease or cirrhosis who were suspected as having HCC were scored "yes". Studies that recruited healthy patients in the control group and groups known to have HCC were scored "no". Studies without sufficient information to make a judgment were scored "unclear".
Acceptable reference standard
Histopathology is the currently acceptable reference standard recommended by EASL for HCC. If histopathology is not available, HCC diagnosis is usually established by ultrasound, MRI or CT when either of them shows a nodule with arterial hypervascularization >2 cm [9
]. Studies using the reference standard consisting of the above mentioned standards were scored "yes". Studies using neither histopathology nor the above imaging modalities were scored "no". Studies without sufficient information were scored "unclear".
Suitable time between reference standard and index test
Blood samples collected before intervention were considered acceptable, knowing that HCC is unlikely to disappear spontaneously. Therefore, studies in which blood samples were collected before intervention were scored "yes". Studies in which blood samples were collected after intervention were scored "no". Studies without sufficient information were scored the study as "unclear".
Sample verification by reference standard
Studies in which all the patients received GP73 and AFP tests and whose disease status was confirmed by the above reference standard were scored "yes". Studies in which some patients missed the above reference standard were scored "no".
Consistency of reference standard
Studies in which the HCC diagnosis was confirmed by the same type of reference standard (histopathology or imaging techniques) in all patients were scored "yes". Studies in which the HCC diagnosis of some patients was confirmed by histopathology while that of some other patients was confirmed by imaging modalities were scored "no". Studies without sufficient information to make a conclusive judgment were scored "unclear".
Reference standard independent of index test
Studies that did not include GP73 and AFP in the reference standard were scored "yes", and studies that included GP73 and AFP in the reference standard were scored "no".
Reference standard blinded
Studies in which blood samples for GP73 and AFP measurement were analyzed by technicians who were blind to the reference standard results were scored "yes". Studies in which blood samples for GP73 and AFP measurement were analyzed by technicians who were aware of the reference standard results were scored "no". Studies without providing sufficient information were scored "unclear".
Index test blinded
Studies in which the disease status was confirmed by the reference standard in all patients without the results of GP73 and AFP were scored "yes". Studies with known GP73 or AFP results were scored "no".
Relevant clinical information
Studies were scored "yes" in which the same clinical data were available when test results were interpreted as would be available when the test is used in practice. Studies with unavailable clinical data in practice were scored "no". Studies without sufficient information were scored "unclear".
Uninterpretable/intermediate test results reported
Studies in which all uninterpretable or intermediate results were reported were scored "yes". Studies without reporting intermediate results were scored "no". Studies without sufficient information to judge were scored "unclear".
Studies in which all details that happened to the patients included were clearly reported were scored "yes". Studies without explaining the reason for withdrawal were scored "no".
Using the 'metandi' module for Stata (version 10), sensitivity and specificity were calculated, and the diagnostic accuracy was summarized for each study. Meta-regression was performed in an attempt to explain the observed heterogeneity. Data were presented as forest plots and receiver operating characteristic curves. Forest plots display the sensitivity and specificity of individual studies with the corresponding 95% confidence intervals. The receiver operating characteristic curves show individual study data points with size proportional to study weight, and the hierarchical summary curve resulting from the hierarchical summary receiver operating characteristic model.