Missing data: not registered vs not available

Solution 1:

register = to put information, especially your name, into an official list or record

Cambridge

To use register is to suggest two possibilities about the values.

First: they could not be registered because there was no information about them (they did not exist).

Second: there was information about the values but it was not registered, in which case readers might wonder why it had not been registered (did you make a value judgement? Are you biased? Were you selective?).

The reader cannot distinguish between these possibilities.

I therefore feel register is not what you are looking for.

If, on the other hand you say not available within the table, this is a plain statement that the required values did not exist in a form that could be used for the paper.

For these reasons I suggest that not available is acceptable and is the better of your two options.

Solution 2:

Your answer is in your question; you should address these values as not recorded.

Doctors use medical records to record things about the patient. Here are some example usages of the verb record as it relates to medical records:

More important for healthcare providers is that the courts tend to consider that if a medical decision, treatment or procedure is not recorded in the clinical notes then it has not been performed.
_{Source: NCBI—How to keep good clinical records}

This process for making this type of correction can be as simple as just letting your doctor know that something was recorded incorrectly so your doctor can change it.
_{Source: Verrywell Health—How to Correct Errors in Your Medical Records}

When adding your more detailed note to the patient's record you should include your name, the date and time of your added note, your findings on examination and an explanation as to why these were not recorded at the time.
_{Source: MDU—Amending patient records}