We experienced a case with a falsely low value by a blood MMP-3 measuring reagent employing a recently structured new latex immunoturbidity. The case involved duplicate orders for one patient in a single day, and the blood collection amounts and measured values were approximately 6.0 mL and 206.6 ng/mL and approximately 1.0 mL and 107.5 ng/mL. The latter MMP-3 concentration was 48% of the former, showing a low tendency. Therefore, an experiment was conducted by adding serum to the blood collection tubes with or without a serum-separating agent of four different manufacturers (Terumo, Sekisui Medical, Nipro, and Becton Dickinson), and similar results as our experienced case were obtained with the Terumo tube with serum-separating agent, which had been used in this case. The amount of whole blood was obtained by conversion assuming a hematocrit value of 40%, and the addition ratio was calculated relative to the predetermined amount of the tube showing a falsely low value. Falsely low values were observed at < or = 56%, < or = 21%, < or = 20%, and , or = 33% for Terumo, Sekisui Medical, Nipro, and Becton Dickinson, with tubes containing a serum-separating agent, and at < or =10%, < or =8%, < or =19%, and < or =14% for Terumo, Sekisui Medical, Nipro, and Becton Dickinson, respectively, with plain tubes. Falsely low values were not observed with the 10-ml plain tube of Terumo and the 9-ml plain tube of Nipro (untreated tube). Based on these results, care should be taken if samples are below the predetermined amount of the blood collection tube to determine the serum MMP-3 by this method.