Specimen:
(1) Urine samples may be spot (usually fasting morning), timed (usually 2-4 hours) or 24 hour collections.
(2) If RIA is used for analysis, radioisotope scans should be avoided prior to specimen collection.
(3) The specimen should be frozen if not tested shortly after collection
total urinary cAMP excretion rate corrected by GFR, as nmol per dL glomerular filtrate =
= (urinary cAMP concentration in µmol/L) * (serum creatinine in mg/dL) / (urinary creatinine in g/L)
Two different equations are given to then calculate the nephrogenous cAMP:
(1) Method in Teitz (1995)
nephrogenous cAMP rate corrected by GFR =
= (total urinary cAMP excretion rate corrected by GFR) - (amount of plasma cAMP filtered into the urine) =
= (total urinary cAMP excretion rate corrected by GFR) - ((plasma cAMP) * (creatinine clearance))
(2) method in Broadus (1979)
nephrogenous cAMP rate corrected by GFR =
= (total urinary cAMP excretion rate corrected by GFR) - (plasma cAMP as nmol/dL)
Normal Range:
• The normal plasma cAMP is 14-26 nmol/L in males and 13-23 nmol/L in females.
• The normal total urinary cAMP is 1.83-4.50 nmol per deciliter glomerular filtrate (18.3-45.5 nmol per liter of glomerular filtrate).
• The normal nephrogenous urinary cAMP: 0.29 - 2.81 nmol per dL glomerular filtrate (2.9-28.1 nmol per liter glomerular filtrate).
Interpretation:
• Measurement of nephrogenous cAMP is of limited use today.
• Measurement of plasma cAMP can be technically difficult. If plasma cAMP cannot be measured adequately, the measurement of total urinary cAMP by itself can be used as a gauge of PTH action.
• Elevated nephrogenous cAMP is seen in 90+% of patients with primary hyperparathyroidism. The test may be of use in patients with hyperparathyroidism with serum PTH levels in the "normal" range.
• Low values for nephrogenous cAMP are seen in patients with absent or suppressed parathyroid function. Decreases in nephrogenous cAMP are seen after parathyroidectomy.
• Increased urinary cAMP excretion is normally seen after PTH infusion. Urinary cAMP excretion after PTH administration can be used to assess PTH resistance. In patients with hypoparathyroidism and in normal patients, urinary cAMP excretion increases 10-20 times in response to parathyroid hormone infusion. In patients with pseudohypoparathyroidism (resistance to PTH), this increase is not seen.
Limitations:
• Many neoplasms are associated with increased urinary cAMP excretion, sometimes due to production of PTH related protein by the tumor.
• Results from patients with hyperparathyroidism can overlap with results from normal subjects. Not all patients with hyperparathyroidism have abnormal results.
