Rennin vs Renin

As part of the tests that I am doing in order to find the cause of my high blood pressure (which finally is reaching normal values, thanks to the heavy drug regime), I had to have (yet another) blood test. The nurse told me that it was for "Renine" (of course, she spoke to me so I didn't see how it was written down) - this was pronounced as if it were a French name that rhymes with Janine.

This sounded strange to me as the only rennin that I know of is used in the making of yoghurt and is derived from a cow's stomach. Of course, I wasn't going to argue with the nurse, who said that the blood has to be taken in the urban medical centre (where I had the covid vaccine) and anyway it requires a 12 hour fast. What could be so special about this test, I wondered?

When I got home, I looked up renin and discovered that it is a hormone that the human body secretes that plays a part in regulating blood pressure - it's good to know that my kidney doctor is leaving no stone unturned (pun unintentional, but it's a good one). When I had the test done, I asked the nurse what was so unusual about this test; apparently the only reason that I had to go to the medical centre was that there was a driver waiting to take my blood (and presumably anyone else who had the same test) to a laboratory in Jerusalem where they must specialise in this rare test.

After a day or two, I was notified of the result: "Less than 1.96". This result has me baffled, primarily because there are no units given. Also, "less than" implies that 1.96 whatevers is some cut off value, but this doesn't sit well with what's written in the Wikipedia entry (not that I trust this). Renin is usually measured as the plasma renin activity (PRA) that is measured specially in case of certain diseases that present with hypertension or hypotension. PRA is also raised in certain tumors. A PRA measurement may be compared to a plasma aldosterone concentration (PAC) as a PAC/PRA ratio. That doesn't help very much.

Moving to plasma renin activity, one reads that Reference ranges for blood tests of plasma renin activity can be given both in mass and in international units (μIU/mL or equivalently mIU/L, improperly shown as μU/mL or U/L, confusing mcU/mL used where Greek μ not available), with the former being roughly convertible to the latter by multiplying with 11.2.

Then a table is presented, which is even more confusing

Unit	Lower limit	Upper limit
ng/(mL*hour)	0.29, 1.9	3.7
μIU/mL	3.3, 21	41

How can one source have a lower limit of 0.29 whereas another source has a lower limit of 1.9 that is 6.5 times higher?? And how does "less than 1.96" fit into this? Is this lower than the lower limit, assuming that the Israeli health ministry has decided to use 1.96 as the lower limit? Does "less than 1.96" mean that the laboratory can't measure less than this, so it's not possible to know how much lower the result is than the lower limit? So many questions, so few answers.

I haven't made a followup appointment yet as I have to have a special ultrasound scan at the end of May, when I am also seeing an urologist, so theoretically I will be in the dark until June. I suspect, however, that I will get a note from my clinic stating the result and my family doctor's understanding of the result.

An ultrasound scan that I underwent a month or so ago shows that there are slightly abnormal results with regard to the bladder (hence the urologist) but these might be normal, considering my age, and may or may not have anything to do with my blood pressure.