Mesothelin is currently the most studied serum biomarker of malignant pleural mesothelioma. To examine the reported diagnostic accuracies and evaluate whether this biomarker can be an adjunct to an earlier diagnosis of mesothelioma, we performed an IPD meta-analysis on 16 published diagnostic studies, representing a total of 4,491 individuals.
On review, all studies had a good methodologic quality but did show large differences in number of participants, clinical characteristics (age, type of control group, mesothelioma stage, and histologic subtype), and reported diagnostic thresholds of mesothelin. In addition, clinically relevant information concerning mesothelioma stage and histology was often not available. Such heterogeneity cannot be adequately addressed in systematic reviews or meta-analyses based on aggregate mesothelin data.41,42
Conducting an IPD meta-analysis allowed us to adequately quantify and address the observed between-study heterogeneity.
First, we evaluated how the diagnostic accuracy of mesothelin compared across studies. Interestingly, even when the large differences in the applied thresholds were eliminated, the sensitivity and specificity of mesothelin still displayed a substantial between-study heterogeneity. To gain more insight in the sources of this heterogeneity, we performed an ROC regression analysis. Before this analysis, mesothelin levels were standardized to account for the previously reported correlation with age43
and between-laboratory differences.15
Subsequent results showed that the between-study heterogeneity in diagnostic accuracy can be explained by differences in type of control group, mesothelioma stage, and histologic subtype. Mesothelin levels better differentiated patients with mesothelioma from controls without a malignancy than from those with lung cancer, whereas controls were better discriminated from patients with advanced epithelioid or biphasic mesothelioma than from those with early-stage or sarcomatoid disease. These results confirmed the findings of individual studies and are the consequence of mesothelin overexpression in some lung cancers, a lack of mesothelin expression in nonepithelioid mesothelioma, and the association of mesothelin levels with tumor burden.9,12
Altogether, studies that include predominantly young healthy controls and older patients with an epithelioid histology and a more advanced disease are likely to report a high diagnostic accuracy of mesothelin, and vice versa.
To confine such over- or underestimation to a minimum, the use of mesothelin in early diagnosis was evaluated in a clinically relevant model, in which symptomatic or high-risk controls were differentiated from patients with stage I or II epithelioid and biphasic mesothelioma. Although the resulting AUC of mesothelin was acceptable, this value merely provides an indication of its overall diagnostic performance and is of little relevance towards its actual use in clinical practice.20
A mesothelin blood test would especially be useful if it allows clinicians to efficiently steer further diagnostic steps and shorten the current diagnostic delay of mesothelioma. Therefore, we evaluated mesothelin in the following two specific clinical settings: as an adjunct to rule in (through a positive blood test) or to rule out (through a negative blood test) the diagnosis of early-stage mesothelioma. For a negative mesothelin test to aid in excluding mesothelioma, the use of a high-sensitivity threshold (typically in the range of 1.00 to 1.50 nmol/L) is a first requirement. However, our results indicated that at a sensitivity of 95% for mesothelioma, more than 75% of the controls would have false-positive test results, leading to an inordinate number of individuals undergoing unnecessary diagnostic work-ups or biopsies. This is especially relevant for mesothelioma, because this malignancy has a low prevalence, even in populations at risk. As a result, a negative mesothelin test cannot serve as an adjunct in excluding mesothelioma diagnosis, even at a high-sensitivity threshold. For a positive mesothelin test to serve as an adjunct to include diagnosis, the use of a high-specificity threshold (likely in the range of 2.00 to 2.50 nmol/L) is typically required. At a specificity of 95%, however, we found that approximately 70% of patients with early-stage epithelioid or biphasic mesothelioma would remain undetected—an unacceptably high proportion. Although a positive mesothelin test at a high-specificity threshold presents a strong incentive to urge ensuing diagnostic steps (eg, thoracoscopy), its poor sensitivity clearly limits the added value to early diagnosis.
Different approaches can be pursued and combined to anticipate this limited accuracy of serum mesothelin. First, clinicians could use sequential mesothelin measurements to monitor symptomatic patients for marked changes in their blood levels, rather than solely rely on a single mesothelin measurement and a fixed diagnostic threshold.44
When comparing the latter approach with a longitudinal algorithm, a recent retrospective study indeed saw an increase in sensitivity for mesothelioma from 16% to 40%.45
Second, accounting for clinical characteristics that affect serum mesothelin levels, like age, glomerular filtration rate, and body mass index, might also improve the performance of this biomarker.43
Third, the current gold standard for the measurement of serum mesothelin, the Mesomark ELISA, should be critically looked at. In addition to challenging this assay with previously developed mesothelin ELISAs,22,23
the development of more sensitive antibodies is also of interest. Fourth, the quest for more accurate biomarker panels should be pursued. Given the heterogeneity of mesothelioma, it is indeed plausible that a single biomarker cannot provide the necessary sensitivity and specificity for clinical practice. However, none of the studied combinations with mesothelin, including megakaryocyte potentiating factor, osteopontin, carcinoembryonic antigen, CA-125, cytokeratins, and hyaluronic acid, so far managed to substantially outperform mesothelin.16,27,32,33,36
Further biomarker research could therefore specifically focus on patients who lack elevated mesothelin levels. For any candidate biomarker (or combination) of mesothelioma, it will be essential to evaluate its accuracy in direct comparison with mesothelin in a sufficiently large study population including relevant controls and patients with early-stage mesothelioma. Serum mesothelin is currently also under investigation in other fields of mesothelioma management, including monitoring therapy response and estimating patient prognosis.43,46–49
It is obvious that further biomarker research is equally relevant for these fields.
Our meta-analysis has some limitations that require consideration. First, IPD was collected from 4,491 individuals, but the ROC regression analyses were performed on smaller groups of participants because of missing data on age, tumor stage, and histologic subtype. Second, cytology was used in a number of studies to diagnose mesothelioma. This approach is typically considered to have a high risk of diagnostic error,2
though experienced centers report reliable results.8
Although the controversy remains, the consequences for our meta-analysis were limited, because only a small number of the patients with mesothelioma (9%) were actually diagnosed with cytology. In addition, these patients lacked data on tumor stage and histology and, therefore, were not included in most of our analyses. Third, we cannot exclude the possibility of a positive publication bias (ie, negative studies on mesothelin that never got published). Fourth, other factors that affect serum mesothelin levels, such as glomerular filtration rate and body mass index,43
were not accounted for, because these were not reported in the original studies. Future studies on mesothelin are strongly encouraged to report all of these clinical characteristics to more efficiently match study participants and interpret the obtained results.
In conclusion, our IPD meta-analysis indentified the presence and the origin of a substantial between-study heterogeneity in the diagnostic accuracy of mesothelin and allowed to evaluate the use of mesothelin in a clinically relevant model. We found that, in symptomatic or high-risk individuals, a negative blood test for mesothelin is not a useful adjunct to exclude mesothelioma, even at a high-sensitivity threshold. Conversely, a positive blood test for mesothelin at a high-specificity threshold was found to be a strong incentive to urge further diagnostic steps and could possibly lead to an earlier diagnosis. However, the associated poor sensitivity of mesothelin for early mesothelioma clearly limits its added value to early diagnosis and emphasizes the need for further biomarker research.