app.alert still shows after age is entered.

Once a checkbox have been checked, an alert message will appear if the age field is empty. 

Click "OK", then setFocus on age field.

After I enter the age and click out of the age box, the message still appears another time, as if the value hasn't been committed yet.

How do I show the message only once?

if (getField("Run").value == "1")
{

if (this.getField("MaxHR").value == "")

app.alert("Please enter patient's age or Date of Birth!",3);

getField("age").setFocus();

}

}