Of 2121 patients identified, 2024 were included for analysis and 97 were excluded. Of the patients included for analysis, 1167 had uncomplicated appendicitis, 355 had complicated appendicitis, 303 underwent a negative appendicectomy and an additional 200 patients were in the comparison discharge group. Of the 97 excluded patients, 19 patients were excluded with tumors or adenomas of the appendix. 36 pregnant patients were excluded. 42 patients with alternative diagnoses were excluded (e.g. intestinal worms).
Demographics, clinical findings and serum biomarker levels are given for all patients as well as those with and without appendicitis (i.e. negative appendicectomy and discharged group combined) in Table 1, in addition to the results of the univariate and multivariate analysis.
Table 1
Appendicitis compared to non-Appendicitis in patients who had an appendicectomy
|
All patients (n = 2024)
|
Appendicitis (n = 1521)
|
Non-Appendicitis (n = 503)
|
Univariate p-value
|
OR, OR 95% CI, multivariate p-value
|
Age (mean years)
|
31.2 ± 16.0
|
32.2 ± 16.6
|
28.1 ± 13.6
|
<0.0001
|
1.011, (1.003, 1.020), p = 0.005
|
Female Gender
|
1096 (54%)
|
733 (48%)
|
363 (72%)
|
<0.0001
|
0.431 (0.339, 0.550), p<0.0001
|
Fever
|
365 (18%)
|
307 (20%)
|
58 (12%)
|
<0.0001
|
1.16 (0.828, 1.165), p = 0.375
|
Tachycardia (above 100)
|
455 (23%)
|
369 (25%)
|
86 (17%)
|
0.001
|
1.015 (0.849, 1.56), p = 0.365
|
Bilirubin
|
16.9 ± 8.9
|
17.9 ± 9.2
|
13.7 ± 7.0
|
<0.0001
|
1.030 (1.013, 1.048), p<0.0001
|
WCC
|
12.2 ± 4.6
|
13.0 ± 4.6
|
9.9 ± 4.0
|
<0.0001
|
0.960 (0.876, 1.053), p = 0.390
|
Neutrophils
|
9.4 ± 4.6
|
10.2 ± 4.5
|
6.8 ± 4.0
|
<0.0001
|
1.233 (1.120, 1.358), p < 0.0001
|
Caption: Demographics, clinical characteristics and mean serum biomarker levels for all patients, those admitted with suspected appendicitis but discharged without an operation. Means with ± one standard deviation are listed for continuous variables. Number of patients and proportion in brackets are given for dichotomous variables. For the multivariate analysis, odds-ratios greater than 1 indicates increased risk of appendicitis. WCC White Cell Count.
Appendicitis versus Non-Appendicitis
Patients with appendicitis were compared to those without appendicitis (i.e., a combination of the negative appendicectomy and discharged without surgery groups). In the univariate analysis, compared to patients with negative appendicectomy, patients with appendicitis were more likely to be male (p<0.0001), older (p<0.0001), febrile (p <0.0001), tachycardic (p = 0.001) and have higher levels of bilirubin (p<0.0001), WCC (p<0.0001) and neutrophils (p<0.0001). In the multivariate analysis, only male gender (OR 0.431, p<0.0001), older age (OR 1.011, p<0.0001) and increased neutrophils (OR 1.233, p<0.0001) and bilirubin (OR 1.030, p>0.0001) were independent predictors of appendicitis.
To ensure the inclusion of complicated appendicitis patients did not skew the above results, the multivariate analysis was repeated comparing non-appendicitis (n = 503) to uncomplicated appendicitis alone (n=1167), excluding complicated appendicitis. Compared to the non-appendicitis group, the independent predictors of uncomplicated appendicitis were male gender (OR 0.4495, 95% CI (0.350, 0.574), p<0.0001), increased neutrophils (OR 1.200, 95% CI (1.089, 1.323), p<0.001) and increased bilirubin level (OR 1.025, 95% CI (1.008, 1.043), p=0.005). Age was no longer significant (p = 0.302). These results show that hyperbilirubinaemia is also an independent predictor of uncomplicated appendicitis.
Receiver operator curve analysis (Figure 1) demonstrated increased neutrophil count (AUC 0.725) to be the most accurate biomarker, for appendicitis, compared to increased WCC (AUC 0.708, p<0.0001) and increased bilirubin (AUC 0.657, p = 0.0001). Bilirubin and WCC did not have significantly different AUC (p=0.28). Therefore, all biomarkers are correlated with an increased likelihood of appendicitis, however increased neutrophil count was the most sensitive and specific test to differentiate appendicitis from those without appendicitis.
Appendicitis versus Discharged Group (Excluding Negative Appendicectomy)
Compared to the group of patients discharged without an operation, the negative appendicectomy group were younger (mean age 25.8 versus 31.5, p<0.0001), febrile at presentation (15% versus 6%, p = 0.002) and tachycardic (21% versus 12%, p = 0.013), There were no significant differences in the neutrophils, WCC, bilirubin or proportion of patients who were female.
Univariate and multivariate analyses were performed comparing patients with appendicitis with the discharged group, excluding the negative appendicectomy patients (Table 2). On univariate analysis, the bilirubin was higher in the appendicitis group, (mean 17.9 versus 14.4, p=0.0002) however, this was not significant in the multivariate analysis (Table 2).
Table 2
Patients with pathologically confirmed appendicitis compared to patients discharged without surgery
|
Appendicitis (n = 1521)
|
Discharged (n = 200)
|
Univariate p-value
|
OR, OR 95% CI, multivariate p-value
|
Age (mean years)
|
32.2 ± 16.6
|
31.5 ± 14.1
|
0.575
|
0.997, (0.987, 1.001), p = 0.586
|
Female Gender
|
733 (48%)
|
151 (76%)
|
<0.0001
|
0.353 (0.247, 0.505), p<0.0001
|
Fever
|
307 (20%)
|
12 (6%)
|
<0.0001
|
2.59 (1.38, 4.86), p < 0.0001
|
Tachycardia (above 100)
|
369 (25%)
|
24 (12%)
|
<0.0001
|
1.403 (0.870, 2.263), p = 0.165
|
Bilirubin
|
17.9 ± 9.2
|
14.4 ± 7.3
|
<0.0001
|
1.013 (0.991, 1.037), p = 0.241
|
WCC
|
13.0 ± 4.6
|
9.8 ± 4.0
|
<0.0001
|
*
|
Neutrophils
|
10.2 ± 4.5
|
6.9 ± 3.9
|
<0.0001
|
1.187 (1.135, 1.241) p < 0.0001
|
Caption: The negative appendicectomy group were more likely to be tachycardic, febrile and younger than the discharged group, so analysis was repeated with these patients excluded. WCC White Cell Count.
* Omitted from logistic regression model due to co-linearity with neutrophils.
Uncomplicated versus Complicated Appendicitis
The results of the univariate and multivariate comparison of uncomplicated versus complicated appendicitis are provided in Table 3. In the univariate analysis, all covariates were significant predictors of complicated appendicitis. The multivariate analysis demonstrated increased age (OR 1.036, 95% CI [1.028, 1.045[, p<0.001), presence of fever (OR 1.685, 95% CI [1.234, 2.300], p<0.001), tachycardia (OR 1.611, 95% CI [1.178, 2.201], p<0.003), increased bilirubin (OR 1.035, 95% CI[1.021, 1.050], p<0.001) and increased neutrophils (OR 1.148, 95% CI (1.036, 1.272), p=0.008) were independent predictors of complicated appendicitis.
Table 3
Uncomplicated Compared to Complicated Appendicitis
|
Uncomplicated (n= 1168)
|
Complicated (n=354)
|
Univariate p-value
|
Odds Ratio (OR), Multivariate p-value and OR 95% CI
|
Mean age in years
|
29.9 ± 0.9
|
39.9 ± 2.1
|
<0.0001
|
1.036 (1.028, 1.045), p<0.001
|
Female Gender
|
584 (50.0%)
|
149 (42.1%)
|
0.009
|
0.804 (0.611, 1.059), p = 0.120
|
Fever
|
194 (16.7%)
|
113 (32.9%)
|
<0.0001
|
1.685 (1.234, 2.300), p < 0.001
|
Tachycardia
|
248 (21.4%)
|
121 (35.2%)
|
<0.0001
|
1.611 (1.178, 2.201), p < 0.003
|
Serum total bilirubin
|
16.74 ± 0.47
|
21.75 ± 1.19
|
<0.0001
|
1.035 (1.021, 1.050), p < 0.001
|
WCC
|
12.5 ± 0.25
|
14.61 ± 0.51
|
<0.0001
|
0.958 (0.868, 1.058), p = 0.400
|
Neutrophils
|
9.23 ± 0.64
|
12.05 ± 0.5
|
<0.0001
|
1.148 (1.036, 1.272), p = 0.008
|
Caption: Characteristics of those with uncomplicated versus complicated appendicitis, with univariate and multi-variate comparison. Baseline characteristics for all patients with appendicitis (n = 1521) have already been given in the second column of Table 1. Means with ± one standard deviation given for continuous variables. Number of patients and proportion in brackets are given for dichotomous variables. An odds ratio of greater than 1 indicated increased odds of complicated appendicitis.
Receiver operator analysis (Figure 2) demonstrated that the AUC for bilirubin (0.6451) and neutrophils (0.6455) were higher compared to WCC (0.621, p<0.001), however the AUC for neutrophils was not statistically significantly different to the AUC for bilirubin (p=0.97). Thus, the sensitivity and specificity of bilirubin for the detection of complicated appendicitis was no better or worse than neutrophils.
Diagnostic Accuracy and Cut-Off Values
Table 4 and Table 5 provides a list of sensitivities, specificities, positive predictive values (PPVs) and negative predictive values (NPVs) for each of the three biomarkers at different cut-off levels for appendicitis versus non-appendicitis and uncomplicated versus complicated appendicitis. The sensitivity, specificity, PPVs and NPVs were relatively poor over a range of values, mirroring the results from the ROC analysis.
Table 4
Diagnostic Accuracy of Biomarkers for Appendicitis versus Non-Appendicitis
|
Appendicitis versus Non-Appendicitis
|
|
Cut-off value
|
Sensitivity
|
Specificity
|
PPV
|
NPV
|
Total Serum Bilirubin
|
4
|
0.999
|
0.0041
|
0.752
|
0.576
|
5
|
0.989
|
0.018
|
0.752
|
0.352
|
10
|
0.87
|
0.270
|
0.782
|
0.408
|
15
|
0.591
|
0.651
|
0.836
|
0.345
|
20
|
0.331
|
0.850
|
0.869
|
0.296
|
25
|
0.174
|
0.935
|
0.890
|
0.273
|
30
|
0.1
|
0.968
|
0.904
|
0.263
|
35
|
0.053
|
0.982
|
0.899
|
0.256
|
WCC
|
1.5
|
1
|
0.002
|
0.751
|
1.000
|
|
5
|
0.984
|
0.047
|
0.757
|
0.493
|
|
10
|
0.728
|
0.577
|
0.838
|
0.413
|
|
12.5
|
0.535
|
0.778
|
0.879
|
0.357
|
|
15
|
0.314
|
0.900
|
0.904
|
0.303
|
|
17.5
|
0.159
|
0.947
|
0.900
|
0.272
|
|
20
|
0.070
|
0.978
|
0.906
|
0.259
|
|
25
|
0.011
|
0.994
|
0.847
|
0.250
|
Neutrophils
|
0
|
1
|
0
|
0.751
|
|
|
2
|
0.993
|
0.020
|
0.753
|
0.486
|
|
5
|
0.860
|
0.397
|
0.811
|
0.485
|
|
7.5
|
0.709
|
0.667
|
0.865
|
0.432
|
|
10
|
0.512
|
0.820
|
0.896
|
0.358
|
|
12.5
|
0.290
|
0.900
|
0.897
|
0.296
|
|
15
|
0.143
|
0.960
|
0.915
|
0.271
|
|
20
|
0.025
|
0.992
|
0.904
|
0.252
|
Caption: Sensitivity and specificity for biomarkers for a representative selection of different cut-off values. The lowest biomarker levels listed were the lowest recorded in the data-set. PPV and NPV are based on prevalence of 75.1% in the sample. PPV Positive Predictive Value, NPV negative predictive value.
Table 5
Diagnostic Accuracy of Biomarkers for Uncomplicated vs Complicated Appendicitis
Uncomplicated vs. Complicated Appendicitis
|
|
Cut-off value
|
Sensitivity
|
Specificity
|
PPV
|
NPV
|
Total Serum Bilirubin
|
4
|
1.000
|
0.001
|
0.233
|
1.000
|
5
|
0.997
|
0.013
|
0.234
|
0.936
|
10
|
0.928
|
0.147
|
0.248
|
0.871
|
15
|
0.731
|
0.452
|
0.288
|
0.847
|
20
|
0.497
|
0.720
|
0.350
|
0.825
|
25
|
0.304
|
0.865
|
0.406
|
0.804
|
30
|
0.185
|
0.926
|
0.431
|
0.789
|
35
|
0.119
|
0.967
|
0.522
|
0.784
|
WCC
|
2.1
|
1.000
|
0.000
|
0.233
|
1.000
|
|
5
|
0.986
|
0.016
|
0.233
|
0.794
|
|
10
|
0.841
|
0.306
|
0.269
|
0.864
|
|
12.5
|
0.655
|
0.501
|
0.285
|
0.827
|
|
15
|
0.430
|
0.722
|
0.319
|
0.807
|
|
17.5
|
0.254
|
0.867
|
0.367
|
0.793
|
|
20
|
0.143
|
0.952
|
0.474
|
0.786
|
|
25
|
0.026
|
0.993
|
0.530
|
0.771
|
Neutrophils
|
0
|
1.000
|
0.000
|
0.233
|
1.000
|
|
2
|
0.997
|
0.009
|
0.234
|
0.905
|
|
5
|
0.946
|
0.166
|
0.256
|
0.910
|
|
7.5
|
0.843
|
0.332
|
0.277
|
0.875
|
|
10
|
0.670
|
0.536
|
0.304
|
0.843
|
|
12.5
|
0.425
|
0.751
|
0.341
|
0.812
|
|
15
|
0.262
|
0.894
|
0.428
|
0.800
|
|
20
|
0.048
|
0.983
|
0.461
|
0.773
|
Caption: Sensitivity and specificity for biomarkers for a representative selection of different cut-off values. The lowest biomarker levels listed were the lowest recorded in the data-set. PPV and NPV are based on prevalence of 30.3% complicated appendicitis. PPV Positive Predictive Value, NPV negative predictive value.
Combinations of biomarkers were assessed to ascertain if there was any improvement in diagnostic accuracy. As the ROC analysis demonstrated that WCC was inferior to bilirubin and neutrophils, only the combination of neutrophils and bilirubin has been performed. Table 6 provides sensitivity and specificity for combined cut-off values over a selected representative range of bilirubin and neutrophils. Compared to Table 4 and 5, combining neutrophils and bilirubin did not markedly improve the sensitivity or specificity.
Table 6
Combinations of Cut-Offs and Sensitivity / Specificity
|
|
Appendicitis vs. Non-Appendicitis
|
Complicated vs. Uncomplicated Appendicitis
|
Cut-Off values
Bilirubin
|
Neutrophils
|
Sensitivity
|
Specificity
|
Sensitivity
|
Specificity
|
5
|
5
7.5
10
15
20
|
84.7
70.1
51.1
14.7
3.29
|
42.4
68.5
82.7
95.4
98.6
|
94.1
83.9
66.4
26.0
5.4
|
18.2
34.1
53.6
88.7
97.4
|
10
|
5
7.5
10
15
20
|
73.0
61.0
44.9
13.4
2.89
|
61.0
86.7
87.5
96.7
99.0
|
85.6
76.6
60.7
24.9
4.5
|
30.7
43.7
59.9
90.2
97.6
|
20
|
5
7.5
10
15
20
|
29.0
25.7
20.0
6.8
2.0
|
91.6
97.0
96.6
98.6
99.2
|
44.9
42.1
33.9
15.3
3.4
|
75.9
79.3
84.2
95.8
98.5
|
30
|
5
7.5
10
15
20
|
10.3
8.8
7.2
3.0
1.0
|
96.4
99.0
98.6
99.0
99.0
|
17.8
16.7
13.8
6.5
1.4
|
92.0
93.6
94.9
98.1
99.1
|
Caption: Selected combinations of cut-off values of bilirubin and neutrophils and corresponding sensitivity and specificity for the two comparisons. White cell count was not evaluated as it was the poorest biomarker in the initial analysis.