Patients in the VitD-deficient group had a 1.82-fold increased chance of acute rejection (82%) compared with patients in the VitD-sufficient group, and this effect was significant (OR 1.82; 95% confidence interval [CI] [1.29, 2.56]; I2 = 52.3%). CI was narrow, which indicated that this result was conclusive and can be generalized to a larger population (Fig. 2).
As it was shown in Fig. 1, primary studies had different results. We performed some subgroup analyses to find potential sources of this statistical heterogeneity. Moreover, subgroup analyses let us analyze studies that are similar together on the aspect of at least one variable. The results of subgroup analyses are summarized in Table 2.
Table 2
The results of all subgroup analyses compared with total result.
Subgroup
variable
|
Category
|
Number of studies
|
Pooled effect size
|
95% CI
|
I2
|
Study
quality
|
High
|
6
|
2.16
|
1.21, 3.87
|
69.5%
|
Low
|
8
|
1.41
|
1.04, 1.90
|
0.0%
|
Induction therapy
|
IT
|
7
|
1.78
|
1.00, 3.16
|
62.7%
|
non-IT
|
7
|
1.82
|
1.17, 2.84
|
36.7%
|
Age in the deficient group
|
≤ 47 years
|
7
|
2.23
|
1.28, 3.86
|
27.7%
|
> 47 years
|
4
|
1.61
|
0.92, 2.83
|
44.1%
|
Age in the sufficient group
|
> 47 years
|
6
|
1.75
|
1.04, 2.96
|
64.3%
|
≤ 47 years
|
5
|
2.40
|
1.00, 5.78
|
37.2%
|
Gender
|
With more male patients
|
5
|
2.85
|
1.31, 6.18
|
12.7%
|
With fewer male patients
|
6
|
1.65
|
0.97, 2.79
|
66.7%
|
Gender in the deficient group
|
With more male patients
|
5
|
2.58
|
1.94, 3.43
|
0.0%
|
With fewer male patients
|
6
|
1.68
|
0.91, 3.09
|
52.3%
|
Gender in the sufficient group
|
With more male patients
|
5
|
3.20
|
1.56, 6.56
|
5.0%
|
With fewer male patients
|
6
|
1.58
|
0.94, 2.65
|
65.2%
|
Donor type in the deficient group
|
Both deceased and living donors
|
7
|
1.79
|
1.12, 2.87
|
57.7%
|
living donor
|
3
|
3.60
|
0.81, 16.11
|
60.4%
|
Total
|
|
14
|
1.82
|
1.29, 2.56
|
52.3%
|
Quality of Studies
High-quality studies were compared with low-quality ones. There were six studies in the high-quality subgroup and eight studies in the low-quality one. High-quality studies showed 2.16-fold increased chance of acute rejection. The results of both high-quality (OR 2.16; 95% CI [1.21, 3.87]; I2 = 69.5%) and low-quality subgroups (OR 1.41; 95% CI [1.04, 1.90]; I2 = 0.0%) were significant and consistent with the overall result (Fig. 4).
Induction Therapy
There were seven studies in each subgroup. The results of both IT (OR 1.78; 95% CI [1.00, 3.16]; I2 = 62.7%) and non-IT subgroups (OR 1.82; 95% CI [1.17, 2.84]; I2 = 36.7%) were consistent with the overall result. The result in the IT subgroup was not significant (Supplemental Fig. 3). This subgroup did not affect the pooled effect size.
Age: In the Deficient and Sufficient Groups
Among 14 studies, 11 studies had age information of both deficient and sufficient groups separately. VitD-deficient patients ≤ 47 years showed a 2.23-fold increased chance of acute rejection (OR 2.23; 95% CI [1.28, 3.86]; I2 = 27.7%), which was significant. VitD-deficient patients > 47 years showed a 1.61-fold increased chance of acute rejection (61%) (OR 1.61; 95% CI [0.92, 2.83]; I2 = 44.1%), which was not significant (Fig. 5).
VitD-sufficient patients ≤ 47 years showed a 2.40-fold increased chance of acute rejection, which was not significant (OR 2.40; 95% CI [1.00, 5.78]; I2 = 37.2%). VitD-sufficient patients > 47 years showed a 1.75-fold increased chance of acute rejection (75%), which was significant (OR 1.75; 95% CI [1.04, 2.96]; I2 = 64.3%) (Supplemental Fig. 4).
Gender
Among 14 studies, 11 studies had gender information of both deficient and sufficient groups separately. Studies with more male patients showed a 2.85-fold increased chance of acute rejection, which was significant (OR 2.85; 95% CI [1.31, 6.18]; I2 = 12.7%). Studies with fewer male patients showed a 1.65-fold increased chance of acute rejection (65%), which was not significant (OR 1.65; 95% CI [0.97, 2.79]; I2 = 66.7%) (Fig. 6).
Moreover, studies with more male patients in the VitD-deficient group showed a 2.58-fold increased chance of acute rejection, which was significant (OR 2.58; 95% CI [1.94, 3.43]; I2 = 0.0%). Studies with fewer male patients in the VitD-deficient group showed a 1.68-fold increased chance of acute rejection (68%), which was not significant (OR 1.68; 95% CI [0.91, 3.09]; I2 = 52.3%) (Supplemental Fig. 5).
Studies with more male patients in the VitD-sufficient group showed a 3.20-fold increased chance of acute rejection, which was significant (OR 3.20; 95% CI [1.56, 6.56]; I2 = 5.0%). Studies with fewer male patients in the VitD-sufficient group showed a 1.58-fold increased chance of acute rejection (58%), which was not significant (OR 1.58; 95% CI [0.94, 2.65]; I2 = 65.2%) (Supplemental Fig. 6).
Donor Type
Among 14 studies, 10 studies had donor type information. Studies with both living and deceased donors showed a 1.79-fold increased chance of acute rejection (79%), which was significant (OR 1.79; 95% CI [1.12, 2.87]; I2 = 57.7%). Studies with living donors showed a 3.60-fold increased chance of acute rejection (OR 3.60; 95% CI [0.81, 16.11]; I2 = 60.4%). However, this result was not significant and conclusive. (Supplemental Fig. 7).
Meta-Regression Analysis
Nine studies reported the rate of eGFR. We performed a meta-regression analysis on eGFR in each group separately. The eGFR of patients did not affect the pooled effect size. In other words, VitD deficiency independent from the eGFR of patients increased the chance of acute rejection (Fig. 7).
Sensitivity Analysis
Sensitivity analysis showed that if the studies are separately omitted, the pooled effect size does not change remarkably. Therefore, the pooled effect size had a robust result (Fig. 8 and Supplemental File 5).