Persistently low lab values vs persistent low lab values?

From the clarification of the question, the problem is to describe a group of patients typified by those satisfy columns 1, 3, and 4 in the first table below

 ------------------------------    
     Start          6 months
    A     B         A     B
 ------------------------------
   low  normal     low  normal
   low  low        low  normal
 --------------------------------

but which excludes others such as those in the second table:

     ------------------------------    
     Start          6 months
    A     B         A     B
 ------------------------------
   low  normal     low  high
   normal  low     low  normal
   low  low        low  high
   low  low        low  low
   low  low        normal  normal
 -------------------------------

This can be described as:

The group in which the level of peptide A remained low for 6 months and the level of peptide B was normal after 6 months.

and in writing this up in a report it would seem necessary to state it precisely in this or a very similar form.

The question is what to do if you have to refer repeatedly to such a group. A construct like “persistent low levels of peptide A-only” is clumsy, ugly and imprecise as it does not include the second condition regarding peptide B. I cannot imagine a phrase that embodies both conditions and would therefore second the suggestion made by @terdon to define them with code names — Group A, Group I, Hypo-A, or whatever.

Gentle suggestions from an old-timer

1. Diagram or tabulate your problem to explain it. It may also help you explain it to yourself.
2. Scientific language is difficult because precision is paramount and this often means using words that are not in everyday use. However when you can use a simple English word, do so. There is nothing clever about replacing plain English by long latinate words.
3. Avoid strings of adjectival nouns (e.g. the ugly and clumsy suggestion of “persistent Peptide A-only-decrease”) by the simple expedient of a preposition. “Of” is not a four-letter word. You may have to use such combinations in the column headings of a table, but that doesn’t mean you need to use them in the text.
4. If you think this will make the sentence too long, you probably need to divide it in two.
5. Make your own decisions on style on the basis of reading clarity — don’t just blindly follow the majority.
6. As a scientist use a precise term such as “concentration” or “amount”, rather than the aqueous “level”.