Q: Which factors may give false positive higher HbA1C level?

Answer: 

Higher than actual  level of HbA1C can be seen in people with a longer red blood cell lifespan, such as with Vitamin B12 or folate deficiency.

Reference:

Kilpatrick ES, Bloomgarden ZT, Zimmet PZ (2009). "Is haemoglobin A1c a step forward for diagnosing diabetes?". BMJ 339: b4432.