A long discussion about non-APS patients have accuracy issues with their finger stick readings, some of which had complications due to those inaccurate readings.

Lupus or antiphospholipid antibody syndrome (APS) may falsely prolong the INR value. Testing with an APS-insensitive laboratory method is recommended for these patients.

The accuracy and precision of POC devices are inconsistent and less accurate when an INR is above the therapeutic range (i.e., INR >3.5) or, for some devices, when a patient has antiphospholipid antibodies.

INRs from POC instruments are unreliable patients with the clotting disorder called antiphospholipid antibody syndrome (APLA syndrome) who are on warfarin [ref 7]. In these patients, the POC devices give INR readings that are too high, or the instruments report error messages. This is the case with any of the 3 instruments on the market. If you have APLA syndrome, your INRs should be checked from blood drawn from a vein and tested in a laboratory. That value can then be compared to the INR obtained with a POC instrument from a finger stick. Only if both values correlate well may it be acceptable for you to use the POC machine for self-testing.

Based on the conclusions of the studies mentioned, different methods of measuring PT/INR behave differently with different APA patients. It is therefore very important to assess the individual patient together with the responsiveness of the PT/INR reagent/system to be used to monitor warfarin therapy (lab or POC). It is also important when performing a correlation study comparing one PT/INR system to another to consider whether any of the patients included in the study have Anti-phospholipid Syndrome. The presence of such samples in the population may affect the correlation.

The error was discovered because of changes to the reagents — a substance that is mixed with a patient's blood to do the test. When the region couldn't get reagent for some aging equipment, senior technologists started to call around to find out how much reagent the rural sites had on hand and the testing error was discovered. About five rural labs need machines replaced. "We're going to be rethinking our equipment needs in the rural sites as a whole," Ledingham said. "Whether each of those five individual machines are going to be replaced or perhaps some of them will be serviced from a central site — it will really depends on the needs of the patients and the clinicians in those communities."

Point-of-care devices should be used with caution for INR determination in APS (Briggs et al, 2008; Perry et al, 2010). Most manufacturers list APS as a specific exclusion to their use.