What are causes of elevated haemoglobin level and haematocritin 6 yr old other than polycythemia vera? i read dehydration is this true?

Elevated?: "Elevated" means different things to different people. Lab tests come back with a result value, that can be compared to a "reference range." The "reference range" is not a "normal range" even though people use the terms alike. The reference range means 95% of the healthy people of that age group get results inside that range; and 5% of normal, healthy people get their results outside that range.