The urine-to-blood pCO2 gradient (U-B pCO2) following an oral bicarbonate load can be used to detect a defect in distal renal tubule acidification. It can aid in the diagnosis of renal tubular acidosis.


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)



• 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.



• A normal person should have a gradient > 20 mm Hg after the bicarbonate load.


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