g full text, 5 studies were excluded based on inclusion criteria. Finally, 6 studies^\[[@R5],[@R14],[@R20]--[@R23]\]^ involving 529 patients were eligible for data extraction and meta-analysis (Fig. [1](#F1){ref-type="fig"}). Of these 529 patients, 208 patients belonged to the PRP group and 321 to the control group.

![Flow chart of the literature search.](medi-96-e7262-g001){#F1}

Table [1](#T1){ref-type="table"} summarizes the characteristics of the 6 included RCTs, which were published between 2009 and 2016. Sample size of the studies was 21 to 315 patients. All of the trials involved primary unilateral TKA, and all patients were diagnosed with knee osteoarthritis except one study,^\[[@R22]\]^ which included the patients with rheumatoid arthritis. All of the included studies were compared with blank control. Baseline characteristics between the two groups in each study were well matched. Two trials^\[[@R5],[@R23]\]^ used spinal anesthesia, one trial^\[[@R5]\]^ involved combined sciatic nerve and femoral nerve block, and the remaining trials did not mention the type of anesthesia used. A pneumatic tourniquet was applied in all the included studies except one study,^\[[@R21]\]^ which did not report the use of a tourniquet. Two studies^\[[@R14],[@R22]\]^ reported use of a drain during TKA, and no postoperative drains were applied in the other 4 studies. Two studies^\[[@R5],[@R21]\]^ described an indication for transfusion associated with a fall in hemoglobin level or hematocrit and clinical symptoms. Variation in the amount of PRP used was noted. Thromboembolic complications such as deep vein thrombosis, pulmonary embolism, and venous thrombus embolism were reported in all studies except 1 study.^\[[@R22]\]^ Outcome of units transfused was reported in 1 study,^\[[@R5]\]^ which was not analyzed owing to insufficient data.

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Characteristics of the included studies.

![](medi-96-e7262-g002)

3.2. Study quality and GRADE of evidence
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Risk of bias in the included studie