An 83-year-old woman with ESRD was having occasional trouble with her dialysis catheter and had intermittently required alteplase-locking. She had been on a stable dose of PO4 binders and her PO4 was very well controlled. However, at one of her visits, her PO4 was noted to be 10.1 (previously 3.5-5) with no change in her diet and the following month it was 35.7mg/dl. Her labs were taken in the usual fashion, after discarding this first 5mls from the catheter. It was postulated that the alteplase might be interfering with the assay for PO4.
In order to confirm this, the authors withdrew 4 samples in 2ml increments from a catheter that had been locked with alteplase during the previous dialysis session. The PO4 concentration in the 4 samples was 161.5, 9.56, 6.47 and 5.68 mg/dl respectively. They then reviewed the records of a patient who had intermittent problems with hyperphosphatemia over the previous year. His mean PO4 level on days on which the catheter was not locked with alteplase was 6mg/dl compared to 9mg/dl on days with alteplase. In retrospect, his PO4 binders were increased inappropriately at least once in response to a spurious test.
So why did this happen? It turns out that the makers of alteplase use phosphoric acid for pH adjustment during the manufacturing process. Although the quantity is very small, it is enough to contaminate a draw from the catheter if it is not adequately removed beforehand. Spurious hyperphosphatemia has also been described in samples contaminated with heparin.