Table 1. The main characteristics of the included studies on DR.
| First author | Year | Study design | Country | No. of patients |
Age mean/median | Male/female | NOS | ||
| Non-DR | NPDR | PDR | |||||||
| Akdoğan[26] | 2016 | Retrospective | Turkey | 158 | 120 | Non-DR: 57.3±12.2 | Non-DR: 59/99 | 7 | |
| DR: 59.8±9.2 | DR: 47/73 | ||||||||
| Ateş[27] | 2009 | Retrospective | Turkey | 30 | 30 | 30 | NR | NR | 5 |
| Buch[29] | 2017 | Retrospective | India | 220 | 80 | NR | NR | 6 | |
| Citirik[30] | 2015 | Prospective | Turkey | 43 | 45 | 52 | Non-DR: 60.4±8.5 | Non-DR: 22/21 | 6 |
| NPDR: 61.4±9.3 | NPDR: 17/28 | ||||||||
| PDR: 59.4±7.2 | PDR: 20/32 | ||||||||
| Demirtas[31] | 2015 | Prospective | Turkey | 240 | 67 | NR | NR | 6 | |
| Dindar[32] | 2013 | Retrospective | Turkey | 36 | 24 | NR | NR | 6 | |
| Güngör[33] | 2016 | Retrospective | Turkey | 50 | 52 | NR | Non-DR: 19/31 | 6 | |
| DR: 18/34 | |||||||||
| Li[34] | 2014 | Retrospective | China | 72 | 67 | 70 | Non-DR: 54.2±9.2 | Non-DR: 35/37 | 7 |
| NPDR: 57.7± 10.0 | NPDR: 34/33 | ||||||||
| PDR: 58.3±9.4 | PDR: 33/37 | ||||||||
| Li[47] | 2013 | Retrospective | China | 103 | 132 | NR | NR | 6 | |
| Li[35] | 2016 | Retrospective | China | 52 | 47 | Non-DR: 55.1±15.2 | Non-DR: 31/21 | 6 | |
| DR: 54.1±10.8 | DR: 26/21 | ||||||||
| Ma[36] | 2017 | Retrospective | China | 20 | 20 | 20 | Non-DR: 57.3±6.5 | Non-DR: 12/8 | 5 |
| NPDR: 60.8±7.3 | NPDR: 8/12 | ||||||||
| PDR: 57.6±7.3 | PDR: 12/8 | ||||||||
| Niu[37] | 2013 | Retrospective | China | 20 | 25 | Non-DR: 46.5±8.3 | Non-DR: 12/8 | 6 | |
| DR: 51.2±8.3 | DR: 13/12 | ||||||||
| Papanas[38] | 2004 | Retrospective | Greece | 89 | 167 | NR | NR | 6 | |
| Radha[39] | 2016 | Retrospective | India | 30 | 14 | NR | NR | 5 | |
| Sheng[40] | 2017 | Retrospective | China | 102 | 102 | NR | NR | 7 | |
| Tetikoğlu[41] | 2016 | Retrospective | Turkey | 63 | 56 | 80 | NR | NR | 6 |
| Tuzcu[28] | 2014 | Retrospective | Turkey | 70 | 64 | 58 | Non-DR: 55.8±10.5 | Non-DR: 38/32 | 6 |
| NPDR: 60.1±8.6 | NPDR: 32/32 | ||||||||
| PDR: 57.5±9.3 | PDR: 31/27 | ||||||||
| Wei[42] | 2017 | Retrospective | China | 94 | 52 | 40 | Non-DR: 58.14±11.93 | Non-DR: 50/44 | 7 |
| DR: 58.42±12.09 | DR: 49/43 | ||||||||
| Xu[43] | 2012 | Retrospective | China | 45 | 40 | NR | Non-DR: 23/22 | 5 | |
| DR: 26/14 | |||||||||
| Yilmaz[44] | 2016 | Prospective | Turkey | 89 | 88 | 86 | Non-DR: 60.9±6.3 | Non-DR: 49/40 | 7 |
| NPDR: 62.7±7.2 | NPDR: 48/40 | ||||||||
| PDR: 61.7±7.9 | PDR: 49/37 | ||||||||
| Yu[48] | 2000 | Retrospective | China | 60 | 40 | NR | NR | 6 | |
| Yue[45] | 2015 | Retrospective | China | 125 | 62 | 59 | Non-DR: 56.00±3.75 | Non-DR: 73/52 | 6 |
| NPDR: 53.50±3.56 | NPDR: 34/28 | ||||||||
| PDR: 56.0±3 | PDR: 28/3 | ||||||||
| Zhang[49] | 2002 | Retrospective | China | 20 | 20 | Non-DR: 58.0±9.0 | Non-DR: 9/11 | 5 | |
| DR: 60.0±1.3 | DR: 8/12 | ||||||||
| Zhou[46] | 2016 | Retrospective | China | 328 | 51 | Non-DR: 57±16 | Non-DR: 198/130 | 6 | |
| DR: 63±15 | DR: 34/17 | ||||||||
DR: Diabetic retinopathy; NPDR: Non-proliferative diabetic retinopathy; PDR: Proliferative diabetic retinopathy; NR: Not reported; NOS: Newcastle-Ottawa Scale.