Hello again,

On last weeks post I talked about three laboratory tests: a white blood cell count, a c-reactive protein (CRP) count, and a erythrocyte sedimentation rate (ESR). As I said in the last post, I have found that these three test are usually the first tests to be ordered when their is suspicion of an infection. But if the results are elevated, doctors can not conclude a patient has an infection quite yet, they must proceed with laboratory tests that tend to be a little more invasive, such as a knee aspiration (aka arthrocentesis). You might be wondering, whats the point of the first three tests if you can’t even use them for diagnosing an infection. Well the point is to confirm a doctors suspicion that the knee is not doing well with the prosthesis, and that much more invasive clinical procedures and laboratory tests need to be performed. 

A knee aspiration is the clinical procedure of using a syringe to draw fluid from the knee. As you can imagine this is much more painful than getting your blood drawn (a blood sample is all that is needed for the CRP, ESR, and WBC count tests). I will go more in detail about knee aspirations and how they pertain to TKA infections after mentioning some trends I have noticed. 

I have seen that a knee aspiration is usually the next step after the CRP, ESR, and WBC count tests. But, sometimes the knee aspiration is the first step, and this could be because the infection is a bit more obvious in that patient, or because the laboratory test results were never imported into the patients chart (missing information is one of the flaws of this project). 

Anyways, the fluid from knee aspirations are sent to a laboratory where the fluid sample is placed in a dish and watched to see if bacteria, fungi, or viruses grow. If these germs are detected, than there is a strong chance of an infection. 

In an earlier post, I mentioned a certain set of criteria many doctors follow when diagnosing an infection. I will include it in this post too, so you don’t have to go back. 

1) There is a sinus tract communicating with the prosthesis; or
2) A pathogen is isolated by culture from at least two separate tissue or fluid samples obtained from the affected prosthetic joint; or
3) Four of the following six criteria exist:

a) Elevated serum erythrocyte sedimentation rate (ESR) and serum C-reactive protein (CRP) concentration,

b) Elevated synovial leukocyte count,

c) Elevated synovial neutrophil percentage (PMN%),

d) Presence of purulence in the affected joint,

e) Isolation of a microorganism in one culture of periprosthetic tissue or fluid, or

f) Greater than five neutrophils per high-power field in five high-power fields observed from histologic analysis of periprosthetic tissue at ×400 magnification.

Number 2 and letter e are both talking about knee aspirations. If a pathogen is isolated it means that something grew in the dish and there is a strong chance of infection. Based on this set of criteria, if a pathogen is isolated from two separate fluid samples, than it is safe to say the patient has an infection. But if only a single fluid sample grows a germ, than three other criteria must be met to diagnose a patient with a TKA infection. 

I’m going to stop here, but In next weeks blog I will go over the last few laboratory tests that are used in diagnosing a TKA infection. 

See you then, Komal.