diseases and diagnostic tests

Prep for USMLE Forum

» Log in | Register

Forum

Step 1

Match

Prep4USMLE Forum » USMLE Step 2 CK Forum » Internal Medicine Forum

diseases and diagnostic tests

Author

14 Posts

cache
Forum Guru

Topics: 130
Posts: 275

06/21/04 - 07:58 PM

For the following pairs of diseases and diagnostic tests, which negative test best RULES OUT the disease in question?
a. Ankylosing spondylitis and negative HLA –B27
b. Rheumatoid arthritis and negative RF
c. Rheumatoid arthritis and Normal CRP
d. SLE and negative ANA
e. Gout and normal serum uric acid levels

asmi
Forum Hero

Topics: 1043
Posts: 4,609

06/21/04 - 08:03 PM

E..

M.A.S
Forum Senior

Topics: 28
Posts: 56

06/22/04 - 10:12 AM

BellKicker
Forum Junior

Topics: 3
Posts: 49

06/23/04 - 03:23 AM

B?

If no RF, it's not RA. By definition it's a seronegative artrhopathy, I think.

___________________
Gotta have heart.

beefoxy
Forum Senior

Topics: 18
Posts: 123

06/23/04 - 03:11 PM

E....

___________________
BeeFoxy

tess
Forum Guru

Topics: 131
Posts: 368

06/24/04 - 07:11 AM

"cache" wrote:
For the following pairs of diseases and diagnostic tests, which negative test best RULES OUT the disease in question?
a. Ankylosing spondylitis and negative HLA –B27
b. Rheumatoid arthritis and negative RF
c. Rheumatoid arthritis and Normal CRP
d. SLE and negative ANA
e. Gout and normal serum uric acid levels

Answer is d. ANA is seen 100% in SLE.

fried water
Forum Junior

Topics: 4
Posts: 57

06/24/04 - 11:21 AM

the answer is E. but ANA is positive in only 97% SLE not 100%. Still the best screening test.. though non-specific but 97% sensitive makes it good to r/o SLE if negative.

Gout can prestent with s. uric acid levels normal or even low at time of attack.. so that doesnt even count nothing towards the diagnosis.

All other tests are so non-specific and non-sensitive.

BellKicker
Forum Junior

Topics: 3
Posts: 49

06/24/04 - 04:01 PM

Agreed. It's certainly not E. (that's what you meant, right?). Uric acid levels are notorious for being normal or even low during attacks, so a normal level doesn't rule out anything (a level shouldn't even be measured during an attack).

I still think it's B because RA patients are RF positive by definition.

If there's no RF, we have ruled out RA and must look consider psoriatic, reactive, ankylosing spondylitis etc.

___________________
Gotta have heart.

cache
Forum Guru

Topics: 130
Posts: 275

06/24/04 - 04:53 PM

the answer mentioned is D.
ANA is the most sensitive test for SLE.

M.A.S
Forum Senior

Topics: 28
Posts: 56

06/25/04 - 09:42 AM

#10

Thanks, but when you mentioned "which negative test best RULES OUT the disease" you intended to say "the most SPECIFIC" not most sensitive, am I wrong?

cache
Forum Guru

Topics: 130
Posts: 275

06/25/04 - 06:33 PM

#11

i should have clarified, sensitive not specific..
:wink:

fried water
Forum Junior

Topics: 4
Posts: 57

06/27/04 - 10:46 PM

#12

sorry! yea i meant that it's certainly not E! Typo wink

i meant it's D. And it is D.

M.A.S., it has to be the most 'sensitive' test to r/o any disease.. not specific. Specific tests are not good to rule out diseases but are to rule out false positives (or the normal).. like for HIV.. the screening test is ELISA, which is very sensitive.. thus good to r/o HIV if it comes negative.. the more specific test is always the next step.. like Western Blot.. a specific test.. which wud r/o any false positives that ELISA might have given.

And yea.. bellkicker, RF is not positive in all cases of RA.. rather it's positive in only appx 70% cases.. so absence of RF doesnt really r/o RA. (But absence of ANA.. kinda r/o SLE except those 3% cases)

So yea answer is D.

BellKicker
Forum Junior

Topics: 3
Posts: 49

06/29/04 - 12:26 PM

#13

"fried water" wrote:
And yea.. bellkicker, RF is not positive in all cases of RA.. rather it's positive in only appx 70% cases..

You're right. I don't know why I thought it was always postive. :oops:

___________________
Gotta have heart.

M.A.S
Forum Senior

Topics: 28
Posts: 56

06/30/04 - 10:59 AM

#14

"M.A.S" wrote:
Thanks, but when you mentioned "which negative test best RULES OUT the disease" you intended to say "the most SPECIFIC" not most sensitive, am I wrong?

"fried water" wrote:
M.A.S., it has to be the most 'sensitive' test to r/o any disease.. not specific. Specific tests are not good to rule out diseases but are to rule out false positives (or the normal).. like for HIV.. the screening test is ELISA, which is very sensitive.. thus good to r/o HIV if it comes negative.. the more specific test is always the next step.. like Western Blot.. a specific test.. which wud r/o any false positives that ELISA might have given.

yes you are right! :| and thank to cache for the good question.

You don't have permission to post.