Please, need help with this

Hello All,

I am currently studying and I came about this question in the kaplan book;

A client is hospitalized with a diagnosis of a-fib. Heparin 5,000 units is ordered every 12 hours to be given sub-Q. The physician orders daily PTT. The result of the client's most recent PTT is 55. Which of the following actions should be taken by the nurse.

1) document the result and administer the heparin

2) withhold the heparin

3) notify the physician

4) have the test repeated.

It says that the normal range for a PT is 20 - 45 seconds and the therapeutic range for a client receiving heaprin, an anticoagulant is 1.5 - 2 times the control or normal level.

How is the PTT then calculated to determine if the client is outside or within therapeutic range?

Thanks a lot