An oral load of bicarbonate is given. This is calculated as 48 mEq per 1.73 square meter body surface area.
oral bicarbonate dose in mEq =
= 48 * (body surface area in square meters) / 1.73
weight of bicarbonate salt to give =
= (bicarbonate dose in mEq) * (molecular weight for salt) =
= weight of mmol of salt in grams
For example, sodium bicarbonate (NaHCO3) has a molecular weight of 84.01, with the bicarbonate representing 72.6% of the total weight.
After the bicarbonate load, the urine pH should be > 7.6.
urine to blood pCO2 gradient =
= (urine pCO2) – (blood pCO2)
where:
• I am not sure what kind of blood sample is needed. According to Tietz, a free flowing capillary blood sample gives a pCO2 close to that of arterial blood, while venous blood has a pCO2 6-7 mm Hg higher than the arterial blood.
• Altitude reduces the pCO2 slightly. I am not sure if altitude affects the renal tubular function.
Interpretation:
• A normal person should have a gradient > 20 mm Hg after the bicarbonate load.
