Explain like I'm 5, 1:40 dilution ratios

[Osteoth]

I am a community PCP and have had multiple patients come in with ANA or RF that is 1:32 or 1:40.

I understand that higher dilution means more likely accurate test, but is there a list of the dilution ratios that are significant for each test? Is it all >1:128 that is important?

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[dozitgetchahi]

I am a community PCP and have had multiple patients come in with ANA or RF that is 1:32 or 1:40.

I understand that higher dilution means more likely accurate test, but is there a list of the dilution ratios that are significant for each test? Is it all >1:128 that is important?

Rheumatology here.

An ANA shouldn’t even be counted as positive unless it is 1:80 or greater. I hate when labs count a 1:40 ANA as positive. Even at 1:80, something like 10-15% of the US population will have a positive ANA of that titer. That’s lots and lots of people who likely don’t have any associated illness. Predictive value goes up with concentration, but at the end of the day associated symptoms etc become a lot more important.

As far as RF goes…a lot of labs are moving away from titers. ACR guidelines are that an RF is unlikely to be consistent with RA unless it is at least 3x ULN. ULN of RFs at many labs out there is 15, meaning that an RF of less than 45 is less likely to be consistent with RA.

(Fun fact: your chance of having an equivocally positive RF goes up with age, especially after age 60 or so. So please don’t send me an 80 year old with joint pain and an RF of 18 with the consult “is it RA?” 99 times out of 100, it’s osteoarthritis.)

 

[Osteoth]

Rheumatology here.

An ANA shouldn’t even be counted as positive unless it is 1:80 or greater. I hate when labs count a 1:40 ANA as positive. Even at 1:80, something like 10-15% of the US population will have a positive ANA of that titer. That’s lots and lots of people who likely don’t have any associated illness. Predictive value goes up with concentration, but at the end of the day associated symptoms etc become a lot more important.

As far as RF goes…a lot of labs are moving away from titers. ACR guidelines are that an RF is unlikely to be consistent with RA unless it is at least 3x ULN. ULN of RFs at many labs out there is 15, meaning that an RF of less than 45 is less likely to be consistent with RA.

(Fun fact: your chance of having an equivocally positive RF goes up with age, especially after age 60 or so. So please don’t send me an 80 year old with joint pain and an RF of 18 with the consult “is it RA?” 99 times out of 100, it’s osteoarthritis.)

Perfect thanks. Most recent lady was 60 who said she's been having long covid for the last 2 years, including joint pains, causing her previous PCP to order ANA, RF etc. I think she has illness anxiety, but are you seeing rheumatic manifestations of long covid? Is an ANA >1:80 with "joint pains" significant if elderly/low risk?