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Discussion Starter · #1 ·
Your patient is a 30 year old women with facial nerve paralysis. She also has fever and headache but doesn't have a stiff neck. On physical examination, she has a circular, erythematous, macular rash on the back of her thigh. You suspect that she has Lyme disease. Of the following tests, which one is the appropriate to order to confirm the diagnosis?

A. Blood culture to grow the organism
B. Stain for inclusion bodies within cells involved in the rash
C. Test for the serum antibody against the organism
D. Dark-field microscopy
E. Doesn't need confirmation, it is a clinical diagnosis
 

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Can I choose two things? :)

That's a tough one - I would certainly get a Lyme titer (C), but I would also start her on doxycycline based on the clinical presentation alone (E). I guess that I'll go by the wording of the last sentence of the stem ("Which one... to confirm the diagnosis?") and choose C. Test for the serum antibody against the organism. What's the best way to answer a question like this?
 

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Discussion Starter · #3 ·
Answer is C.

There is a teaching point i want to bring out in this Q. When do we order serology to screen or confirm diagnosis of infectious diseases?

1. When the organism cannot be cultured, e.g., syphilis and hepatitis A, B, and C.
2. When the organism is too dangerous to culture, e.g., rickettsial diseases,francisella.
3. When culture techniques are not readily available, e.g., HIV, EBV.
4. When the organism takes too long to grow, e.g., Mycoplasma.

One problem with this approach is that it takes time for antibodies to form, e.g., 7–10 days in the primary response. For this reason, acute and convalescent serum samples are taken and a 4-fold or greater rise in antibody titer is required to make a diagnosis. By this time the patient has often recovered and the diagnosis becomes a retrospective one. If a test is available that can detect IgM antibody in the patient's serum, it can be used to make a diagnosis of current infection. In certain infectious diseases, an arbitrary IgG antibody titer of sufficient magnitude is used to make a diagnosis.

We also do serology to confirm old resolved infections. For example, to confirm pharyngitis in a patient with rheumatic fever. By that time pharyngitis is resolved and we can not do rapid strep test.

Yes, Mondoshawan! you are right we would give doxy right away, seeing the severity of the disease, but we also need to confirm and Q is asking about that. Choice E says no need for confirmation.
 

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I just read it in CMMRS .Lyme disease is caused by Borrelia burgdorferi. Diagnostics include,
1.Elevated levels of antibodies against the organism can be detected by ELISA.
2.Western Immunoblotting.

So I will go with "C".???
 

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I think E is the answer. when we have erythema migrans our diagnosis is based only on clinics. we start 21 days therapy.

1. erythema migrans shows up several days to weeks after the tick bite.
2. blood test - it may take four to six weeks for the human immune system to make antibodies against Borrelia burgdorferi.
3. patients with the Lyme rash usually have a negative blood test.
4. patients with other clinical manifestations such as Lyme arthritis will usually have a blood test.
 
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