Prevention of adverse drug events
caused by potassium phosphate injection
From the March 13, 1996 issue
PROBLEM: A 71 year-old patient presented to the emergency
department with atrial fibrillation. A phosphate level measured
0.9 mg/dL (normal serum inorganic phosphate in adults is 3
to 4.5 mg/dL). A nonspecific dose of potassium phosphate was
prescribed. It was ordered as "one amp of potassium phosphate".
The hospital pharmacy did not provide 24 hour service, so
a nursing supervisor retrieved a 15 mL "single dose" vial
of potassium phosphate from the pharmacy. Since the vial and
pharmacy storage bin both cautioned that dilution was necessary,
the nurse diluted the drug in 15 mL normal saline and eventually
administered intravenously at a rate of 1 mL/min. The patient
suffered a cardiac arrest within 4 minutes. Resuscitation
This patient received 8.8 mEq of potassium and 6 mmol of
phosphorus in 4 minutes! Had the entire syringe been administered
he would have received a total of 45 mmol of phosphorus and
66 mEq of potassium!!
SAFE PRACTICE RECOMMENDATION: Concentrated potassium
phosphate injection is a combination of monobasic and dibasic
salt, the ratio of which is dependent on pH. To avoid confusion
it is generally prescribed in terms of millimoles of phosphorus
rather than amount of potassium phosphate. Unfortunately,
this may lead one to overlook the accompanying amount of potassium.
Each mL contains 3 mmol of phosphorus and 4.4 mEq of potassium.
It is very important that the rate of injection and total
amount of phosphate be guided by this. The patient's ECG and
electrolytes must also be monitored. Sodium phosphate injection
is also available. Similarly, when this form of phosphate
is used for replenishment, the amount of sodium must also
When using phosphate injection for electrolyte replenishment,
each institution should have specific guidelines established
based on the level of inorganic phosphate and other clinical
factors such as age, renal function, reason for use, etc.
Phosphorus doses of 0.08 mmol/kg to 0.32 mmol/kg have been
recommended for patients with normal renal function. This
amount is infused over 12 hours and repeated every 12 hours
until serum phosphate is greater than 2 mg/dL (Vanetta JB,
Andress DL, Whang R et al. High dose intravenous phosphorus
therapy for severe complicated hypophosphatemia. Southern
Med J 1983;76:1424-26).
It is also important to dilute phosphate appropriately, and
administer it slowly. This recommendation stems from several
reports of severe complications related to the rate of administration.
Shackney et al (Shackney S, Hasson J. Precipitous fall in
serum calcium, hypotension, and acute renal failure after
intravenous phosphate therapy. Ann Intern Med 1967; 66: 906-16)
described the development of oliguria, hypocalcemia, hypotension,
and death in two patients, one of whom received 100 mmol in
3 hours and the other 50 mmol in 1 hour. The first patient
received 0.56 mmol/min; the second received 0.83 mmol/min.
The patient above received 1.5 mmol/min. In general, the minimum
amount of time that phosphate injection should be administered
is over six hours (Stoff JS. Phosphate homeostasis and hypophosphatemia.
Am J Med 1982;72:489-95).
There were other factors that gave rise to this serious adverse
drug event. Ordering the dose as "one amp" is dangerous since
it is available in 5, 10, 15, 30 and 50 mL sizes. The drug
should only be ordered specifically in millimoles of phosphorus.
Also, depending on the size and scope of service of a hospital
it should be considered a standard of practice that either
the hospital have 24 hour/day availability of pharmacy service
or that a carefully determined supply of needed medications
be available in a suitable after hours storage location. Finally,
the designation "single dose vial" is considered ambiguous
by some practitioners. USP currently has under consideration
a proposal to use the term "single use vial" as an alternative
for these and other vials designated "single dose vial". [D,N,P,Q]