I- Sample is taken on Sodium citrate.

II- Perform PTT for patient, if normal do not proceed with the test.

III- Make a stock dilution of PTT reagent as follow ( 100 µ  reagent +           400 µ  saline). 

IV- 

         Tube 1                                             Tube 2                                 Tube 3

        Control                                              Patient                                  Mixture

  100 µ  control plasma                       100 µ  patient plasma              50µ control plasma      

                                                                                                        +  50 µ patient plasma

V- Add 100  of diluted PTT reagent in each tube then mix for 3 minutes in water bath.

VI- Add 100  of CaCl2 in each tube then start stop-watch.

VII- Calculate the formula:

                     Mixture  -    Controle

LAC =      ----------------------------------             If    >   15 ----à   Positive test

                                 Patient