Diagnosis
4
Table 3. Laboratory Indicators of Proximal Tubular
Dysfunction
Abnormality
Definition of
Abnormality Comment
Serum abnormalities
Hypokalemia Serum potassium
concentration
below laboratory
reference range
• Trend is of more clinical relevance
than single abnormal value
• Neither sensitive nor specific
Low serum
bicarbonate
Serum bicarbonate
concentration
below
laboratory reference
range
• Trend is of more clinical relevance
than single abnormal value
• Nonspecific in the setting of reduced
GFR
Hypophosphatemia Serum phosphorous
concentration
below
laboratory reference
range
• Normal value does not exclude urinary
phosphorous wasting. However,
hypophosphatemia combined with
urinary wasting is diagnostic of renal
tubular injury
Urine abnormalities
Urine glucose
on dipstick
Glycosuria in the
absence of diabetes,
or in diabetics with
well-controlled
blood glucose
• Diagnostic of renal tubular injury
Fractional excretion
of phosphate
(FE
phos)
<10% is normal and
>20% is abnormal
• Increased FE
phos
in the setting of
normal kidney function is of more
clinical relevance than when kidney
function is impaired
• Upward trend in urinary phosphate
wasting is suggestive of tubular injury
• Loss of phosphate may lead to
osteomalacia
