We don't use APP - we assess in the following ways (for most children):
Reading comprehension tests, e.g. QCA or NFER Nelson test sheets which give the child a passage to read and a variety of questions to read and answer in writing. This is done half termly in our school.
Hearing children read, normally as part of a guided reading group, or sometimes individually, depending upon the stage they're at. By listening to the fluency of their reading and discussing their understanding of the book (plot, language, characterisation, patterns, layout, punctuation etc) we can ascertain how they're getting on and identify any very specific needs. This happens every day.
Reading age tests - personally I don't like reading age tests and think they can be very misleading, it's a blunt tool IMHO. However, the head likes them.
We then introduce specific reading interventions for children who need more support, or make decisions about how to develop a child's reading (e.g. progressing through book bands, becoming a 'free reader' or introducing a particular type of text, e.g. playscripts to develop expression in reading dialogue, or dyslexia friendly typeface, or books for reluctant readers).
HTH.