Ascitic fluid analysis

The college loves portal hypertension and ascites. They have asked about these conditions several times (though in the last six years or so it has been neglected). SAQs from past papers include the following:

• Question 28.5 from the first paper of 2022
• Question 5.3 from the first paper of 2020 (does this fluid look infected to you?)
• Question 14 from the second paper of 2001 (information derived from ascitic fluid analysis)

No single paper exists which would say authoritatively, "this is all the possible things you could do with a sample of ascitic fluid", which is in essence the topic of this chapter. A FOAM resource which comes close is Mike Cadogan's LITFL page on this subject, and the time poor exam candidate should probably go there first, as it has LITFL-grade authority and is as comprehensive as it is brief. For something from a peer-reviewed publication, one could turn to an ancient 1982 article by Ward, which is of a high quality, but unfortunately paywalled by Taylor & Francis. Tarn & Lapworth (2010) is also excellent, but free.

The results and their meaning will be discussed here in an order which is similar to what is presented in Question 5.3 from the first paper of 2020, where the trainees were invited to interpret the ascitic fluid of a patient with an "exacerbation of chronic liver disease".

Appearance of ascitic fluid

It has been a long time since a physician might need to carefully scrutinise the appearance of ascitic fluid with the expectation that they will then be able to make accurate management decisions regarding the patient on the basis of this inspection. These days, such clinicians are nearly extinct. Berner et al, in their 1981 editorial for the BMJ, probably represented the last of this group. "A lot may
be learnt from the naked-eye appearance of the fluid" they say, offering no references for what follows:

."..bloodstaining suggests trauma or carcinoma, chylous fluid is found in lymphatic obstruction, and the diagnosis is confirmed if the chylomicrons separate into the supernatant fluid; turbidity indicates infection; bile staining is associated with trauma, pancreatitis, gall stones, neoplasia, and recent biliary surgery; and clear, straw-coloured fluid occurs in parenchymal liver disease."

Just to add a layer of added paternal authority, it must be added that Charles Berner was publishing articles about the analysis of ascites back in 1964, so this really was a man who has seen a lot of ascitic fluid. One might expect somebody like that to taste the sample thoughtfully for a moment and then to accurately report its cell count. 

Realistically, how accurate is the gross appearance of ascitic fluid? Unsurprisingly the greatest interest in answering this question has come from resource-poor environments. According to Aminiahidashti et al (2014), the appearance of a clear, straw-coloured fluid does not rule out SBP. Among their cohort of patients, of those with a nice normal-looking fluid sample, 29% had cytological evidence of infection.

pH of ascitic fluid

This test is so obscure that the author's local laboratory seems to automatically cancel the sample, assuming that the test is so preposterous that nobody could have intentionally ordered it. In the rare situation where one is so interested in the result that one actually rings the laboratory, one may be able to produce a situation where the pH is in fact reported, but by methods no more sophisticated than a quick dunk of a urinalysis dipstick.

What is the point of this? Well. It is another biomarker of spontaneous bacterial peritonitis.  Specifically, an acidic pH is said to be associated with SBP. Bacteria (eg. E.coli, E.faecalis) in the ascitic fluid change the pH by secreting acid metabolites (lactate, acetate, formate, succinate), and this change is apparently reasonably reliable.  For instance, Gitlin et al (1982) found that a pH of less than 7.31 was associated with SBP in their series. Unfortunately, they achieved this level of second decmal point accuracy by using a blood gas analyser. This is not usually available either, as the local Radiometer representative assures the author that any attempt to funnel anything but blood through the modern devices will void their warranty. 

Cell count of ascitic fluid

Erythrocyte count in ascitic fluid defines the ascites as "haemorrhagic" if the count is in excess of 10,000 cells per μl. That is enough to make the fluid look identifiably pink. What does that mean? Nothing specific, it seems; investigators have not been able to associate this finding with any specific nastiness form which it might be a sensitive or specific marker. Also, it probably means nothing good, ie. one would not receive it as a joyously positive feature. Urrunaga et al (2013) found that these patients, when compared to people with non-haemorrhagic ascites, had a much greater risk of being dead after one month of follow-up, as well as a higher risk of being found to have SBP and hepatocellular carcinoma.

An RBC may not be reported because all the blood in the sample had clotted during transport and processing.  The presence of micro-clots in the fluid suggests that the RBCs were there, but then because of some mysterious influence they all clotted and the real RBC count cannot be reported. "Mysterious" is because peritoneal fluid, conventionally, is not supposed to clot very much, because it contains very little clotting protein (Pattinson et al, 1981). The presence of clots suggests that some sort of unpleasant procoagulable process is afoot, and moreover that the peritoneal capillaries are much more porous than they should be, and are releasing clotting factors into the peritoneal cavity (which normally wouldn't happen, as they are all much too large). All of that sounds a lot like malignancy. However, after half an hour of googling, the author cannot confirm any of this, as there does not appear to be anything written anywhere about this finding. As such, this entire paragraph consists of nothing but speculation. 

Leucocyte count should be interpreted in terms of whether or not the culture and gram stain had turned up any bugs, and how many different species there were. This table was reconstructed from  the 2009 article by Koulaouzidis and Such & Runyon (1998):

Range of Possible Ascitic Fluid Findings

Culture	WCC <250\mm3	WCC > 250/mm3
Negative	Normal ascitic fluid	Culture-negative neutrocytic ascites
Single organism	Monomicrobial  nonneutrocytic bacterascites	Spontaneous bacterial peritonitis
Multiple organisms	Polymicrobial bacterascites	Secondary bacterial peritonitis

Other cells could potentially sneak into the ascitic fluid, and their abundance could alert you to the presence of something hideous:

• An excess of lymphocytes  could lead to the diagnosis of lymphoma. 
• An unexpected abundance of macrophages could mean HIV.
• Ascitic eosinophilia could be associated with eosinophilic oesophagitis.

Ascitic fluid protein

The protein content of normal cirrhosis-assciated ascitic fluid is the consequence of plasma protein being solute-dragged across the leaky peritoneal capillary membrane along with ultrafiltered water, pushed out along a concentration gradient. This should produce a low protein:fluid ratio, just like a transudative pleural effusion. However, if something untoward was taking place (i.e. a mesenteric tumour or bacterial infection causing the capillaries to become even more leaky), the fluid protein content should increase. The ascitic protein level can therfore be useful as an alarm signal, letting you know that you need to look for intraperitoneal unpleasantness.

In classical history, the total ascitic fluid protein had been the usual measured variable; but more recently, albumin measurement became popular, and more recently still the serum-ascites albumin gradient (SAAG) had become the standard. It is a gradient, rather than a ratio, and is calculated by subtracting the ascites albumin from the serum albumin:

SAAG = (albumin concentration of serum) – (albumin concentration of ascitic fluid)

The result is reported (at least locally) as g/L or g/dL.  Anything over 11g/L identifies portal hypertension as the cause of ascites (Pare et al, 1983); i.e. if your serum albumin is 20 (as many of the ICU patients would be), the ascitic albumin concentration should be below 9. Any albumin concentration which is more serum-like would suggest that something terrible is happening, but it would not tell you exactly what that is. A high ascitic albumin content could be equally likely a sign of SBP, malignancy, tuberculosis, and so forth. This is quite sensitive; in fact even acute occlusion of the portal vein produces ascitic fluid which has a SAAG of under 11 (Spaander et al, 2010)

Ascitic fluid LDH

The lactate dehydrogenase content of ascitic fluid is another biomarker which has little diagnostic specificity, other than being able to separate ascites into the categories of "cirrhosis" and "something worse". When the cutoff is left at 400 U/mL, its sensitivity at picking up peritoneal malignancy is around 70% (Banerjee et al, 2011, and Ekpe et al, 2018). Back in the day, one would also do a serum LDH level and then compare the ratio (a'la Light's criteria). Boyer et al (1978) determined that an ascites/serum LDH ratio of more than 0.6  was often seen in patients with tuberculous or bacterial ascites, as well as pancreatitis and malignancy.

A range of other tests which you could conceivably order

In some outlandish scenario where your practice is not scrutinised very carefully and everybody working with you is too junior to question your decision-making, what sorts of crazy tests could you order on an ascitic fluid sample? 

• Amylase/lipase - to consider pancreatitis as the cause of ascites
• Gram stain - for bacteria
• India ink stain - for hyphae
• ZN stain for acid-fast bacilli (tuberculosis)
• Culture/sensitivities
• Cytology - presence of malignant cells
• Triglycerides - elevated in chylous ascites
• Alpha-foetoprotein - to investigate for HCC
• Creatinine (is it actually urine?)