Exudative v/s transudative ascites: differentiation based on fluid echogenicity on high resolution sonography.

HM Malde, RD Gandhi 
 Dept of Radiology, KEM Hospital, Parel, Bombay, Maharashtra.

Correspondence Address:
H M Malde
Dept of Radiology, KEM Hospital, Parel, Bombay, Maharashtra.

Full Text

::   Introduction

Cirrhosis and abdominal tuberculosis are amongst the commonest causes of ascites in our country. As ultrasonography (USG) is the primary radiological investigation in these patients, the ability to distinguish transudative from exudative ascites using this modality, would obviously simplify the further diagnostic work-up and management off these patients. An attempt was made using current, high resolution sonographic equipment, to distinguish exudative from transudative ascites based on the echogenicity of the fluid.

::   Methods

Fifty-two patients with ascites (42 males and 10 females; mean age - 37 yrs) were subjected to real-time sonography using one of the new generation high resolution ultrasound equipment (Sonoline AC, Siemens Inc. Germany). A routine (3.5 MHz) sector transducer was followed by a high frequency (7.5 MHz) transducer, to compare the echogenicity of the ascites with the echogenicity of the normal abdominal viscera, and based on this, these fluid collections were graded as:

Grade I : anechoic (may show few internal echoes due to floating particulate matter).

Grade II : hypoechoic as compared to the liver and spleen.

Grade III : echogenicity similar to or slightly greater than that of the liver and spleen.

The sonographic appearance of the fluid collection in each patient was then compared with the findings of diagnostic aspiration.

::   Results

Twenty-eight of the 52 patients reveafed a transudate on biochemical examination of the fluid obtained at diagnostic aspiration. The echogenicity of the ascitic fluid on sonography in each of these 28 patients was of Grade I.

The other 24 patients had an exudative collection as determined by the results of diagnostic aspiration. Twenty-one of these 24 patients had shown on sonography, echogenicity of Grade II or Grade III, while in 3 of 24 ascitic fluid showed a Grade I echogenicity.

A statistical analysis of our results show that an ascitic fluid echogenicity of Grade I has a sensitivity of 90.32%, specificity of 100%, positive predictive value of I and a negative predictive value of 0.875 in diagnosing transudates. An ascitic fluid echogenicity of Grade II or Grade III has a sensitivity of 87.5%, specificity of 100%, positive predictive value of 1 and a negative predictive value of 0.903 in diagnosing exudates.

::   Discussion

The evaluation of a patient with ascites requires that the cause of the ascites be established. A useful framework for the work-up starts with an analysis of whether the fluid is an exudate or a transudate. This distinction is necessary even when the cause of the ascites seems obvious[1]. Diagnostic paracentesis thus, remains the investigation of choice for the routine evaluation of a patient with ascites.

Over the past 2 decades, sonography has revolutionized the work- up of these patients. Apart from guiding diagnostic aspirations, especially of small fluid collections, USG also frequently demonstrates the causative factor responsible for their occurrence (e.g. cirrhosis, hepatoma etc.). However, the utility of sonography in differentiating transudates from exudates has remained variable[2],[3],[4],[5]. Several ancillary findings have been suggested including the presence of fixed or mobile fibrinous strands and septations, fluid loculation, mesenteric thickening with adherent small bowel loops, hepatomegaly with or without distinct hepatic metastases, splenomegaly, lymphadenopathy, ileal wall thickening and diffuse thickening of the greater omenturn for differentiating the exudates ascites from a transudative one[2],[3],[4],[5]. However, they are unfortunately absent in a significant proportion of these patients. In fact, a recent study on a large number of patients with tuberculous peritonitis of the wet- ascitic type[3] reports that none of these findings were present in 52% of patients with an exudative ascites. This clearly outlines the need for more sensitive sonographic criteria to aid the differentiation of exudative from transudative ascites. In addition, we feel that the poor resolution of the older US equipment along with the complex and subjective sonographic criteria used by previous investigators compounded these difficulties.

We conclude from our present study, using the newer generation of high resolution US equipment, coupled with simple and objective grading criteria, that the echogenicity of the ascitic fluid can be a very useful predictive factor in differentiating exudates from transudates.

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