Wednesday, September 30, 2015

Volume of ascitic fluid, abdominal assessment in ascites and my viva incident

Hey everyone!

A colleague asked this question on the study group  - What volume of ascitic fluid are the following tests positive: Puddle sign, Shifting dullness, Fluid thrill?

What I found out - Minimum amount of fluid required for detection of ascites by various methods are:
Diagnostic tap 10 - 20 ml
Ultrasonography 100 ml
CT scan 100 ml
Puddle sign  120 ml
Shifting dullness 500 ml
Fluid thrill 1000 - 1500 ml
I would also like to add an incident that happened in my viva:

I had a patient with liver cirrhosis in my exam. The examiner asked me that which physical examination signs I knew for the detection of suspected ascites and what my findings were.

I said, "I know about the puddle sign, shifting dullness and fluid thrill. I did not elicit the puddle sign because it is difficult for the patient to lie prone and then go on all four extremities."

I knew at the back of my head that puddles sign can be elicited with 100 ml fluid so with confidence, I added, "But it must be positive because in my patient, shifting dullness and fluid thrill is positive."

The examiner exclaims, "I don't know how students come to these assumptions!"
Then he asks, "Do you know the terms sensitivity and specificity?"

He tells me that puddle sign has very little sensitivity, so it may not be produced, irrespective of the ascitic fluid volume.

So my take home point that day was - We are so busy in learning numbers that we forget what is actually seen clinically. That's what the examiners go after in vivas.
My friend argued (On the study group) that since puddle sign detects ascites even in with a fluid volume of 100 ml, it should be the most sensitive test.

Forget clinicals, I needed numbers to see if my examiner right. This is what I found online: 

Sensitivity and specificity of physical signs for ascites:

Bulging flanks
Sensitivity 0.78 
Specificity 0.44

Flank dullness
Sensitivity 0.94 
Specificity 0.29

Shifting dullness 
Sensitivity 0.83 
Specificity 0.56

Fluid wave
Sensitivity 0.50 
Specificity 0.82

Puddle sign 
Sensitivity 0.55 
Specificity 0.51 

So my examiner was right. We assume that sensitivity is - even if there's a little chance of having the disease, it'll get detected. But it doesn't mean that if there's little fluid in ascites, and if a test is capable of detecting it, it is a sensitive test. Sensitivity is TP/TP+FN and that's why, while comparing different tests, sensitivities differ.

That's all!

Conclusion: DO NOT SAY, "But it must be positive" in your viva. Ever :P

-IkaN

(Reference: Cattau EL et al. The accuracy of the physical examination in the diagnosis of suspected ascites. JAMA 1982; 247:1164-6)

5 comments:

  1. Replies
    1. Really awesome
      BT can u plz elaborate wht is tp/tp+fn

      Delete
    2. I don't know but I will get back to you!

      Delete
    3. Probably true positive/to plus false negative .

      That's the formula used to calculate sensitivity in a 2x2 table

      Delete

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