Abnormal serum phosphate (hypophosphataemia or hyperphosphataemia)

Fasting laboratory tests in the morning

• Serum phosphate (repeat)
• Serum calcium
• Serum albumin
• Serum creatinine with eGFR
• 25-hydroxyvitamin D (reimbursed) (persistent hypophosphataemia: osteomalacia suspected)if neededConfirmed hypophosphataemia without obvious shift cause: hyperparathyroidism secondary to vitamin D deficiency (sun, intake, malabsorption) is one cause ^[2]. The NABM reimburses 25-hydroxyvitamin D only if rickets or osteomalacia is suspected, among others ^[3]. Write the indication (suspected osteomalacia) on the prescription; outside this suspicion, the test would be “NR” (not reimbursable in France) ^[3].
• PTH, on the same sample as the serum calcium (if serum calcium high-normal or raised)if neededHypophosphataemia with high-normal or high serum calcium: consider primary hyperparathyroidism ^[2] (see the Hypercalcaemia page). The NABM limits PTH to the diagnosis of parathyroid adenoma, among others ^[3]. Here, the question asked is indeed the diagnosis of a parathyroid adenoma: PTH reimbursed, without “NR” (not reimbursable in France) ^[3].

If persistent hypophosphataemia with no obvious cause (TmP/GFR)

• The same fasting morning: serum phosphateif needednon-French guidelineUnexplained chronic hypophosphataemia: TmP/GFR (maximum tubular reabsorption of phosphate relative to GFR) looks for renal phosphate wasting; it is calculated on the 2nd morning fasting void, collected at the same time as the blood ^[2].
• Serum creatinineif needednon-French guidelineUnexplained chronic hypophosphataemia: TmP/GFR (maximum tubular reabsorption of phosphate relative to GFR) looks for renal phosphate wasting; it is calculated on the 2nd morning fasting void, collected at the same time as the blood ^[2].
• And on the 2nd morning voiding: urinary phosphateif needednon-French guidelineUnexplained chronic hypophosphataemia: TmP/GFR (maximum tubular reabsorption of phosphate relative to GFR) looks for renal phosphate wasting; it is calculated on the 2nd morning fasting void, collected at the same time as the blood ^[2].
• Urinary creatinine (TmP/GFR calculation)if needednon-French guidelineUnexplained chronic hypophosphataemia: TmP/GFR (maximum tubular reabsorption of phosphate relative to GFR) looks for renal phosphate wasting; it is calculated on the 2nd morning fasting void, collected at the same time as the blood ^[2].

Fasting laboratory tests in the morning

• Serum phosphate (repeat, on a non-haemolysed sample)
• Serum creatinine with eGFR
• Serum calcium
• Serum albumin
• Serum protein electrophoresis (if unexplained hyperphosphataemia with normal renal function)if needednon-French guidelineHyperphosphataemia without renal failure or obvious cause: a monoclonal immunoglobulin (myeloma) can distort the colorimetric phosphate assay (pseudohyperphosphataemia); in a case review, almost all hyperphosphataemias reported in myeloma were artefacts ^[9].
• Serum magnesium (if serum calcium low)if neededHyperphosphataemia with low serum calcium and normal renal function (possible hypoparathyroidism): hypomagnesaemia is a curable cause of functional hypoparathyroidism (PPIs in particular) ^[10].
• PTH, on the same sample as the serum calcium (if low serum calcium with normal renal function)if neededHypocalcaemia with hyperphosphataemia and normal renal function: a low or inappropriate PTH makes the diagnosis of hypoparathyroidism ^[10]. Outside thyroidectomy and dialysis, PTH falls outside the NABM indications: “NR” (not reimbursable in France) ^[3]. See the “Hypocalcaemia” page.
• PTH, reimbursed (after thyroidectomy, or dialysis patient)if neededAfter thyroidectomy (diagnosis and follow-up of hypoparathyroidism) or in dialysed patients, PTH falls within the NABM indications of code 0983: do not write “NR” (not reimbursable in France); in other situations, use the previous line ^[3].