We measure PT and PTT tests with using sysmex (CA50) or Coalaser (DiaLAB) analysers using Biolab reagents. Both instruments measure the tests according turbidometric method. We encounter a "NC error" (PT>60 sec and PTT>180 sec) for about each 20 tests in non turbid patient's plasma. However, when we measure these samples with manual methods or by other instrument even with turbidometric method ( ACL analyser) and using the same reagents, the test results is normal/ mild to moderate increase. How could resolve this problem?