Article Text

Diagnostic accuracy of cryptococcal antigen test in pulmonary cryptococcosis: a protocol for a systematic review and meta-analysis
  1. Binghui Liang1,2,
  2. Zehui Lin3,
  3. Jiachun Li3,
  4. Rongbin Jiang3,
  5. Weijie Zhan3,
  6. Xiaoyun Jian3
  1. 1Department of Respiratory Medicine, The Eighth Clinical Medical College of Guangzhou University of Chinese Medicine, Foshan, Guangdong, China
  2. 2The Second Clinical Medical College of Guangzhou University of Chinese Medicine, Guangzhou, Guangdong, China
  3. 3Department of Respiratory Medicine, Foshan Hospital of TCM, Foshan, Guangdong, China
  1. Correspondence to Dr Zehui Lin; lzhys2018{at}


Background The cryptococcal antigen (CrAg) test was proposed as a rapid diagnostic tool to identify cryptococcal meningitis in patients suffering from AIDS. Several studies have demonstrated its diagnostic performance in cryptococcal meningitis. However, the diagnostic performance of the CrAg test in serum or bronchoalveolar lavage fluid in patients with pulmonary cryptococcosis remains uncertain. Therefore, the purpose of this systematic review is to summarise the evidence concerning diagnostic performance of the CrAg test in patients with pulmonary cryptococcosis.

Methods and analysis Databases such as PubMed, EMBASE, Cochrane Database of Systematic Reviews, Web of Science,, International Clinical Trials Registry Platform, Wanfang Database and China National Knowledge Infrastructure will be searched systematically. The titles and abstracts will be reviewed by two independent reviewers. The Quality Assessment of Diagnostic Accuracy Studies 2 tool will be used to evaluate the risk of bias and clinical applicability of each study. Potential sources of heterogeneity will be investigated through visual inspection of the paired forest plots and summary receiver operating characteristic plots. The pooled summary statistics for the area under the curve, sensitivities, specificities, likelihood ratios and diagnostic ORs with 95% CI will be reported.

Ethics and dissemination The underlying study is based on published articles thus does not require ethical approval. The findings of the systematic review and meta-analysis will be published in a peer-reviewed journal and disseminated in various scientific conferences and seminars.

PROSPERO registration number CRD42022373321.

  • Clinical Decision-Making
  • Respiratory infections
  • Systematic Review

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See:

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.


  • A comprehensive subgroup analysis and sensitivity analysis will be conducted as part of this review.

  • A thorough and robust evaluation of bias will be carried out to ensure the validity of the results.

  • It is possible that there may be variations in the quality of reporting of diagnostic performance measures.

  • This systematic review will only include papers published in either English or Chinese.


Target condition and index test

Pulmonary cryptococcosis is an infectious lung fungal disease caused by major species complexes, Cryptococcus neoformans or Cryptococcus gattii, which can affect both non-immunocompromised (IC) patients and IC patients with AIDS, malignancy or organ transplants and others receiving iatrogenic immunosuppression.1 2 The central nervous system (CNS), lungs and blood are common locations for cryptococcal infection, and cryptococcal meningitis (CM) is a serious illness associated with significant morbidity and mortality. Accordingly, for non-IC patients with pulmonary cryptococcosis, lumbar puncture is recommended to rule out asymptomatic CNS involvement.3–5 Recently, pulmonary cryptococcosis has become more common around the world, particularly in non-IC patients.6 7 Early identification of pathogens in patients with pulmonary cryptococcosis allows for early diagnosis and treatment, as well as a reduction in the rate of misdiagnosis and disease progression.8 However, the majority of patients with pulmonary cryptococcosis have common clinical symptoms, such as coughing with sputum production, fever, dyspnoea and so on. Moreover, the manifestations of chest high-resolution CT image in patients with pulmonary cryptococcosis are also protean, spanning from an isolated lung mass to multiple pulmonary nodules or a widespread disseminated interstitial infection. Consequently, it is very difficult to distinguish the presentations from the many other causes of pneumonia in such patients.9 10 The diagnosis of pulmonary cryptococcosis most often relies on the isolation of cryptococcus from bronchoalveolar lavage fluid (BALF) culture or histopathological examination results,1 9 11 which take time and samples are not readily available, potentially delaying diagnosis and treatment. Cryptococcal antigen (CrAg) testing of serum or cerebrospinal fluid samples is crucial for the rapid diagnosis of patients with CM. A positive result on the CrAg test may indicate the presence of a cryptococcal infection, even if the patient does not have any visible symptoms. This is because the antigens are produced by the fungus and can be detected in the bloodstream before symptoms appear. The CrAg test has been reported to confer high specificity and sensitivity in CM.12–14 Nevertheless, the diagnostic performance of the CrAg test in serum or BALF in patients with pulmonary cryptococcosis remains unclear. Moreover, false-positive and false-negative results have been caused because of many factors such as rheumatoid factor, cross-antigens of other fungi or bacteria and unknown non-specific proteins.15 The purpose of this systematic review is to summarise the currently available data concerning the diagnostic performance of the CrAg test in patients with pulmonary cryptococcosis.

Clinical pathway

In cases of suspected pulmonary cryptococcosis, the CrAg test can be performed on a patient’s blood sample or BALF sample to detect its presence. Because the presence of a positive serum CrAg titre implies deep tissue invasion and a high likelihood of disseminated cryptococcal disease. A study published in 2015 showed that pre-emptive fluconazole treatment of CrAg-positive patients resulted in markedly fewer cases of CM.16 For patients with clinically highly suspected pulmonary cryptococcosis and positive results of CrAg test, pre-emptive antifungal treatment, histopathological and cytological examinations, are recommended. Although pre-emptive treatment based on positive CrAg test and clinical presentation can improve patient outcomes, false-positive results should not be ignored.17 It is common to have a negative CrAg test in blood or BALF during pulmonary cryptococcal infection. Therefore, even if a patient has a high clinical suspicion of pulmonary cryptococcosis, a negative CrAg test result does not rule out the possibility of infection.18


Protocol registration and reporting

The protocol for this systematic review and meta-analysis was registered on the International Prospective Register of Systematic Reviews (PROSPERO) with registration number CRD42022373321. This review protocol is related to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) Protocols 2015.19 The findings of this review will be reported in accordance with the PRISMA of Diagnostic Test Accuracy Studies (PRISMA-DTA) statement.20

Information source

Primarily, electronic databases and PROSPERO will be searched to check for similar published or ongoing systematic reviews to avoid duplication. A systematic search will be conducted using the following electronic bibliographic databases to retrieve peer-reviewed articles: PubMed, EMBASE, Cochrane library, Web of Science,, International Clinical Trials Registry Platform, Wanfang Database, China National Knowledge Infrastructure. In addition, the reference lists of the retrieved studies will be manually scanned to identify relevant articles. An update search will be conducted before we publish this systematic review.

Search strategy

The electronic databases search will use the following keywords and medical subject headings: pneumonia, lower respiratory tract infection, pulmonary, cryptococcus, cryptococcosis, cryptococcus, cryptococcal, cryptococcal capsular polysaccharide antigen, CrAg, cryptococcal capsular antigen. For the detailed search strategy, please refer to online supplemental file 1. We will only include peer-reviewed data and exclude conference abstracts.

Inclusion criteria

The analysis will only include studies that meet the following inclusion criteria: (1) The purpose of the study is to evaluate the diagnostic performance of CrAg in blood or BALF. (2) The study population comprises patients who are clinically suspected of having pulmonary cryptococcosis. (3) Blood or BALF samples were collected from the patients before intervention. (4) The criterion standard for the diagnosis of pulmonary cryptococcosis is established by histopathological review or microbiological examination. (5) The article must have been published in English or Chinese in a peer-reviewed journal. (6) The article must provide data on the number of true-positive, false-positive, true-negative and false-negative cases. The inclusion of studies was not limited to specific HE4 assay methodology, and lateral flow immunoassay (LFA) and ELASA studies will be included.

Exclusion criteria

Studies will be excluded if they meet any of the following criteria: (1) abstracts, review articles and/or lecture presentations; (2) all or some subset of patients in the study did not have reported histopathological diagnosis, microbiological culture or molecular detection and (3) duplicate publication of the same study.

Data management

Articles from all comprehensive searches of databases will be exported as EndNote files (including titles and abstracts), which will then be imported into EndNote as a single library. Duplicate articles found in the searches will be verified and removed.

Study selection

Titles and abstracts of all obtained articles will be independently reviewed by two authors. No matter where there is a disagreement between the two reviewers regarding study inclusion, they will resolve it by discussion and consensus. If consensus cannot be reached, a third author will participate and make the final decision whether to include the article or not. All reasons for the exclusion of articles will be noted, and the review process will be presented using the PRISMA flow chart figure 1.

Figure 1

Flow diagram of the study selection process.

Patient and public involvement

Patients or the public will not be directly involved in the design or planning of the study.

Assessment of methodological quality

The methodological quality of the included studies will be evaluated using the Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) tool.21 We modified the QUADAS-2 by carefully considering the specific diagnostic test being evaluated. Specifically, in domain 1: patient selection, we added an item: Was a retrospective study avoided? And in domain 3: reference standard, we changed the item ‘Is the reference standard likely to correctly classify the target condition?’ into the item ‘Did the study used pathological examination, microbiological culture or molecular detection as a reference standard?’ For the detailed items, please refer to online supplemental file 2. Each of the selected studies will be independently evaluated by two authors. A disagreement among the reviewers on individual items will be resolved by a consensus. Results are presented in the text and in a graph.

Data extraction

We will retrieve the following data from included studies using a standardised data extraction form specifically developed for this systematic review and meta-analysis. The following data will be retrieved: (1) data for constructing the 2×2 tables: number of true positives, false positives, true negative and false negatives; (2) study characteristics: first author, year of publication, study design (type of study, cross-sectional study design, cohort study design or case–control study design), country or region, publication’s language, inclusion and exclusion criteria, diagnostic criteria of cryptococcal pneumonia (reference standard); (3) index test characteristics: type of test principle (LA, ELISA or LFA), types of biological samples used (serum, BALF); (4) QUADAS‐2 items and (5) any other information of relevance (eg, funding source). If key information is missing, we will try to contact the primary authors through email for the missing data. We will resolve disagreements by discussion.

Statistical analysis and data synthesis

Statistical analyses will be conducted following the Cochrane Handbook for Systematic Reviews of DTA (available at We will carry out a graphical descriptive analysis of the included studies. The accuracy of the diagnostic test by creating a 2×2 table for each study based on information extracted directly from the papers will be summarised. The forest plots (sensitivity and specificity separately, as well as the 95% CIs) will be reported. If sufficient studies and data are available, subgroup analyses will be performed based on the type of test principle and the type of specimen. In addition, we plan to assess the effect of risk of bias of included studies on diagnostic accuracy through a sensitivity analysis by excluding studies classified as having a high or unclear risk of bias in at least one of the domains of QUADAS-2. The analyses will be performed using R software. If more than one threshold was reported in a study, we will select the most often used threshold to incorporate in the meta-analysis. However, the specific cut-off value for a positive test result of CrAg test performed by ELISA can vary between different ELISA kits, as it is determined by the manufacturer of the test kit and based on the results of a large number of tests performed on samples known positive and negative results. We will perform the hierarchical summary receiver operating characteristic (ROC) model and calculate the point estimate of either sensitivity or specificity, as well as its corresponding 95% credible interval.

Investigations of heterogeneity

Sources of heterogeneity will be investigated by visually inspecting the paired forest plots and ROC plots. One of the main sources of heterogeneity in diagnostic accuracy systematic review and meta-analysis is the use of different thresholds. We will evaluate the presence of a threshold effect using the Spearman’s correlation coefficient between the logit of sensitivity and the logit of (1−specificity). Heterogeneity between different studies may also result from variation in specimens and test types. If sufficient studies and data are available, we will address the following sources of heterogeneity by adding variables to the meta-regression model: type of test (ELISA, LA and LFA), type of specimen provided (serum and BALF), study design, country. An analysis of subgroup based on the above variables will be performed.

Assessment of publication bias

If there are enough studies, we will generate the funnel plot to assess the possible publication bias. With Deeks’ asymmetry test, where that p<0.05 will be considered a significant asymmetry.22

Grading the quality of evidence

The quality of the included studies will be evaluated using the Grading of Recommendations Assessment, Development and Evaluation working group methodology for diagnostic tests and strategies.23 The quality of evidence will be evaluated across the domains of risk of bias, consistency, directness, precision and publication bias. The quality of evidence will be categorised as high, moderate, low or very low.

Reporting this review

The systematic review will be reported in accordance with the PRISMA-DTA statement with a flow chart highlighting the article screening process.20

Potential protocol amendment

If the protocol is updated, the date and details of the amendment will be saved and used to update the study protocol and the PROSPERO registration record.


The tests used in cryptococcosis should be highly sensitive to ensure the timely initiation of induction antifungal therapy.24 Furthermore, the high cost and difficult monitoring and management of currently recommended treatment, such as amphotericin B, require these tests to be highly specific.25 However, relying on conventional culture and histopathological examination for diagnosis requires an appropriate laboratory environment, trained technicians and sustainable equipment, which may delay diagnosis and treatment. The CrAg test detects the cryptococcal capsular polysaccharide antigen in serum or body fluids, which is disseminated during infection and serves as a biomarker of cryptococcosis. Over the past half-century, increasing interest in the CrAg detection has led to the development of commercial tests based on antibody–antigen interactions, using LA assays, ELISA and FLA.26–28 Systematic reviews indicate that it demonstrates very high accuracy in biological samples for diagnosing CM in high-risk patients, making it a rapid diagnostic method for CM.29 30 However, there is no universal agreement on its diagnostic accuracy of for distinguishing the pulmonary cryptococcosis from the other types of pneumonia. In addition, there is a lack of comprehensive evidence on its diagnostic performance in various clinical samples for pulmonary cryptococcosis. Therefore, this systematic review will summarise the evidence on the ability of the CrAg test to diagnose pulmonary cryptococcosis. Understanding its diagnostic performance will facilitate the its use as a critical diagnostic tool in clinical and public health practice.

Ethics and dissemination

The underlying study is based on systematic reviews of published articles, and therefore, does not require ethical approval. The results of the systematic review and meta-analysis will be published in a peer-reviewed journal.

Ethics statements

Patient consent for publication


Supplementary materials

  • Supplementary Data

    This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

  • Supplementary Data

    This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.


  • Correction notice This article has been corrected since it was first published. Affiliations have been updated.

  • Contributors BL and ZL conceived and designed the study, ZL and WZ completed literature search and drafted the protocol, RJ and JL participated in designing the study and critical revision of the manuscript, XJ took part in the critical revision of the manuscript. All authors read and approved the final manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.