**3. Results**

A total of 132 potentially eligible articles were identified using our search strategy. After the exclusion of 93 articles based on title and abstract for clearly not fulfilling inclusion criteria on the basis of the type of article, study design, population or outcome of interest, or due to being duplicates, 39 articles were left for full-length review. Eighteen of these were excluded from the full-length review as they did not report the outcome of interest, while six articles were excluded because they were not observational studies. Thus, 15 studies (10 cohort studies [58–67] with a total of 431 KTx recipients and 5 cohort studies [68–72] with a total of 108 living kidney donors) were included. The literature retrieval, review, and selection process are demonstrated in Figure 1.

**Figure 1.** Outline of our search methodology. Abbreviation: KTx, kidney transplant.

#### *3.1. Serum Klotho after Kidney Transplantation*

The characteristics of the included studies assessing serum klotho after kidney transplantation are presented in Tables 1 and 2. After KTx, there was a significant increase in serum klotho levels in recipients (at 4 to 13 months post-KTx) in reference to baseline levels before KTx with a mean difference (MD) of 243.11 pg/mL (three studies; 95% CI 67.41 to 418.81 pg/mL, I2 = 93%), Figure 2A. There were significant reductions in serum PTH and phosphate levels with MDs of −134.65 pg/mL (95% CI −176.09 to −93.21 pg/mL, I2 = 0%) and −2.81 mg/dL (95% CI −3.46 to −2.16 mg/dL, I2 = 97%), respectively. There was no significant change in serum calcium levels with a MD of 0.37 mg/dL (95% CI, −0.05 to 0.79 mg/dL, I2 = 83%). Although KTx recipients had lower serum klotho levels with a MD of = −234.50 pg/mL (five studies; 95% CI −444.84 to −24.16 pg/mL, I2 = 93%, Figure 2B) compared to healthy unmatched volunteers, one study demonstrated comparable klotho level between KTx recipients and eGFR-matched controls [66]. Two studies demonstrated high serum klotholevelsin deceased donors as a prognostic marker for good allograft function within one year after KTx (*p* < 0.05) [59,60].



Abbreviations: eGFR, estimated glomerular filtration rate; iFGF23, intact fibroblast growth factor-23; KTx, kidney transplant; N/A, not available.



*J. Clin. Med.* **2020**, *9*, 1834



**Figure 2.** (**A**) Change in Serum Klotho in KTx Recipients after Kidney Transplant. (**B**) Serum Klotho in KTx Recipients Compared to Unmatched Healthy Volunteers.

#### *3.2. Serum Klotho after Living Kidney Donation*

The characteristics of the included studies assessing serum klotho after kidney transplantation are presented in Tables 3 and 4. A total of 108 living kidney donors were identified from five cohort studies. After kidney donation, there was a significant decrease in serum klotho levels post-nephrectomy (day 3 to day 5) with a mean difference (MD) of −232.24 pg/mL (three studies; 95% CI −299.41 to −165.07 pg/mL, I2 = 0), Figure 3A. At one year following the kidney donation, serum klotho levels remained lower than baseline before nephrectomy with a MD of = −110.80 pg/mL (two studies; 95% CI −166.35 to −55.24 pg/mL, I2 = 5), Figure 3B.

**Figure 3.** Changes in serum klotho after living kidney donation: (**A**) immediate post-donation and (**B**) one year post-donation.




**Table 4.** Characteristics of the included studies comparing serum klotho between living kidney donors and healthy volunteers.

There was no significant change in serum FGF-23 at one year post-donation with a MD of = 8.19 pg/mL (two studies; 95% CI −14.24 to 30.62 pg/mL, I2 = 85%), Figure 4A. Compared to unmatched healthy volunteers, living kidney donors had lower serum klotho levels with a MD of = −92.41 pg/mL (two studies; 95% CI −180.53 to −4.29 pg/mL, I2 = 44%), Figure 4B.

**Figure 4.** (**A**) Changes in Serum FGF-23 at one year post-donation and (**B**) Serum klotho levels in kidney donors compared to unmatched healthy controls.

#### *3.3. Evaluation for Publication Bias*

A funnel plot was not drawn because of the limited number of studies in each analysis. Generally, tests for funnel plot asymmetry should be used only when there are at least ten study groups, because the power of the test is too low to distinguish chance from real asymmetry [82]. Egger's regression test demonstrated no significant publication bias in all analyses (*p* > 0.05).
