Characteristics of the included studies
We identified 1001 related articles, of which 104 studies were potentially suitable. 45 studies were eliminated because they provided neither FeNO nor blood NO data. 1 study was excluded due to lack of control subjects. 5 studies were ruled out because these studies contained different kinds of cancer patients, and the data of lung cancer could not be obtained. In addition, 3 repeated studies were also discarded. Finally, 50 studies including 2551 cases and 1691 controls met the including criteria (figure1). There were 40 case-control studies, in which 6 studies were about FeNO, while 34 studies showed blood NO data. 7 studies provided FeNO data, while 15 studies provided blood NO data collected before and after treatment, respectively. The study characteristics were listed in Table 1-4. Lung cancer patients were diagnosed by operation or biopsy. Frequency-matched controls to the cases by sex, age and smoking status were applied in some studies. The treatment regimen mentioned in these studies included operation, chemotherapy, radiotherapy, immunotherapy and traditional Chinese medicine. The scores of included studies ranged from 5 to 9 by NOS.
FeNO level between lung cancer patients and control subjects
There were 6 case-control studies about FeNO level. The FeNO level in lung cancer patients was much higher than that in control subjects (SMD 3.01, 95% CI 1.89-4.13, p < 0.00001) (figure 2a).
FeNO level between pretreatment and posttreatment
7 observational studies reported FeNO level detected before and after therapy. As a result, no significant difference of FeNO level was observed between pretreatment and posttreatment (SMD -0.04, 95% CI -0.46-0.38, p > 0.05) (figure 2b). However, FeNO level elevated in NSCLC subgroup (SMD 0.28, 95% CI 0.04-0.51, p = 0.02) and long-term subgroup (SMD 0.36, 95% CI 0.11-0.61, p = 0.005). In other subgroups, there was no evident variance of FeNO level (table S1).
Blood NO level between lung cancer patients and control subjects
34 case-control studies provided data of blood NO level derived from both lung cancer and control groups. Generally, blood NO concentration in lung cancer patients was higher than that in control subjects (SMD 1.34, 95% CI 0.84-1.85, p < 0.00001) (figure 3a). Subgroup analysis indicated this effect existed in almost all subgroups divided by nationality, sex, age and smoking status. However, this effect was only observed in NSCLC subgroup (SMD 2.40, 95% CI 1.76-3.03, p < 0.00001), while in subgroup containing SCLC patients, there was no difference of blood NO level between lung cancer patients and control subjects (SMD 0.38, 95% CI -0.25-1.02, p = 0.23). The similar phenomenon was also found in subgroups divided by cancer stage. In the subgroup including both early and advanced stage patients, the blood NO level in patients was higher than that in control subjects (SMD 1.60, 95% CI 0.92-2.27, p < 0.00001). For other subgroups (early stage, advanced stage and unknown stage), the difference did not exist (table S2).
Blood NO level in lung cancer patients with different pathological types and stages
A total of 16 studies reported the blood NO level of lung cancer patients with different pathological types respectively, including squamous cell carcinoma (SCC), adenocarcinoma (ADC), large cell carcinoma (LCC) and small cell lung cancer (SCLC) (figure 4a). To compare the blood NO level in each cancer type, network meta-analysis based on Bayesian random-effect model was applied. As a result, blood NO level in each cancer type except for SCLC was higher than that in control patients. There was no significant difference of blood NO level among four kinds of lung cancer patients (figure 4c). The model ranked each pathological type by their relative effect (probabilities of being higher). According to these results, pathological type with the higher probability of being ranked first is LCC, with a probability of 67%, followed by other three kinds of lung cancer, whose probability ranged from 10% - 12%. Cumulative probability of being the pathological type with highest blood NO level was calculated and the cumulative ranking curve of each pathological type was obtained to calculate the SUCRA. According to SUCRA results, LCC (SUCRA=83.5%) presented as the cancer type with highest blood NO level and followed in order by ADC (58%), SCC (56.5%), SCLC (50%) and control (1.5%) (table S3). The node-splitting approach allowed for the identification of two inconsistent nodes (SCC vs. SCLC, ADC vs. SCLC and LCC vs. SCLC). No inconsistent results of direct and indirect comparisons were observed in these three pairs of comparisons (figure 4e). In addition, Brooks-Gelman-Rubin plot illustrated that the NMA model presented good convergence.
There were 19 studies about the blood NO level of lung cancer patients in different stages (early or advanced) (figure 4b). The NMA model revealed that the blood NO level in advanced stage but not early stage was higher than that in control subjects (figure 4d). The model ranked each stage by their relative effect, and advanced stage was ranked first, with a probability of 91%. According to SUCRA results, patients in advanced stage (SUCRA=95.5%) had the highest blood NO level and followed in order by early stage (52.5%) and control (2.5%) (table S4). The node-splitting approach allowed for the identification of two inconsistent nodes (early stage vs. advanced stage). No inconsistent results of direct and indirect comparisons were observed between these two groups (figure 4f). In addition, Brooks-Gelman-Rubin plot illustrated that the NMA model presented good convergence.
Blood NO level between pretreatment and posttreatment
15 observational studies including 18 cohorts reported blood NO level detected before and after therapy. As a result, no significant difference of blood NO level was observed between pretreatment and posttreatment (SMD -0.36, 95% CI -1.08-0.36, p > 0.05) (figure 3b). However, blood NO level decreased in NSCLC subgroup (SMD -0.95, 95% CI -1.89-0.00, p = 0.05) after therapy. In other subgroups, there was no evident variance of blood NO level (table S1).
Publication bias
Publication bias was tested using Begg's and Egger's tests. These tests did not show significant results in comparisons of FeNO. However, Begg’s test and Egger's test showed significant results in the comparisons of blood NO level between cases and control subjects, and between pretreatment and posttreatment, respectively (Table S5). The distribution of data points revealed asymmetry. (figure 5). These results indicated the possibility of publication bias.