We analyzed results from the External quality Control of diagnostic Assays and Tests program to assess current clinical laboratory practice and performance of different methods for factor XIII (FXIII) testing internationally. FXIII proficiency testing data from all eight surveys conducted in 2010 and 2011 were analyzed (1,283 results), comparing the three available methods for detecting FXIII deficiency, thus including clot-solubility qualitative activity, quantitative activity, and antigen. Clot-solubility qualitative assays detected a deficiency in only 16% (11/69) of samples with less than 2% FXIII. Assays using added thrombin detected more deficiencies (33%) than did assays without added thrombin (11%). The most commonly used quantitative activity method tended to produce higher results for low FXIII samples than other quantitative activity methods. Antigen results generally showed good accuracy compared with expected levels. The mean interlaboratory coefficients of variation showed wide variability, especially for samples with less than 10% FXIII activity. Laboratory self-classification of results (as normal vs. abnormal) was good, and was slightly better for specimens with ≤ 25% FXIII than for specimens with 26 to 70% or those with >70% FXIII. We conclude that quantitative activity assays perform better for detecting FXIII deficiency than clot solubility assays, although some quantitative activity assays overestimate low FXIII levels.