Background: Although several tests are used to screen for the presence of LA, none detects all its types. The shortening of APTT observed when the pre-incubation period is prolonged, proved to be a sensitive test for the presence of LA. Material and methods: We determined the APTT, performed with a 4 or 15 min preincubation period (APTTs and APTT15 respectively), in 22 healthy subjects, 3 commercial positive controls for LA, 16 patients with a previous diagnosis of LA and 54 patients with recurrent fetal loss and/or infertility. Evidence of LA was established by a positive Staclot-LA test. Results: APTTs and APTT15 were 31.5±4.7 and 28.4±4.5 seconds respectively in samples from 22 normal subjects. The figures in samples with LA, were 71.5±20.3 s and 58.6±18 s respectively. The difference between the two APTTs performed on an individual sample was defined as the APTT 4-15 and was 2.6±2.0 in normal subjects 2.5±2.8 in 13 patients anticoagulated with warfarin, -10.0±6.5 in 13 patients receiving heparin, and 13.2±4.9 in 15 patients with LA. The test values for LA patients were significantly higher than those for normal subjects (p <0.0001). For values over 5, the APTT 4-15 had 93.3% sensitivity and 100% specificity. In one patient with recurrent fetal loss or infertility, who was LA positive, the APTT 4-15 was positive with a value of 14. Conclusions: This modified TTPA is easy to perform, and provides a reasonably discriminatory value for the presence of LA. Therefore, we recommend the TTPA 4-15 to screen for LA (Rev Méd Chile 2004; 132: 407-12)