Interpreting Ascitic Fluid

Sending ascitic fluid is easy.
Interpreting it correctly is what actually changes management.

This guide focuses on how to read ascitic results quickly and safely on the ward, especially when ruling out SBP and identifying the cause of ascites.

✅ First rule (most important)

Before anything else, always ask:

Is this SBP?

Because:

  • SBP kills
  • It’s common
  • It’s easily treated
  • Missing it is dangerous

Everything else is secondary.

Step 1 – Look at the cell count FIRST

This is the most important result.

🔴 Neutrophils ≥ 250 cells/mm³

👉 SBP → treat immediately

Do NOT wait for:

  • cultures
  • consultant review
  • repeat tap

Start:

  • IV antibiotics
  • Albumin
  • Escalate

This is a medical emergency.

🟢 Neutrophils < 250

SBP unlikely
→ move on to next steps

Step 2 – Look at appearance (quick clues)

Sometimes you get clues before the lab even reports.

Clear/straw coloured

→ typical cirrhotic ascites

Cloudy/turbid

→ infection (think SBP)

Bloody

→ traumatic tap or malignancy

Milky

→ chylous ascites (lymphatic obstruction/malignancy)

Never rely on appearance alone — always confirm with labs.

Step 3 – Calculate SAAG (for cause of ascites)

If trying to work out why the patient has ascites:

Formula:

SAAG = Serum albumin – Ascitic albumin

SAAG ≥ 11 g/L (≥1.1 g/dL)

👉 Portal hypertension

Most common:

  • Cirrhosis
  • Alcohol-related liver disease
  • MASLD
  • Cardiac failure
  • Budd–Chiari

In NHS wards → usually cirrhosis

SAAG < 11 g/L

👉 Not portal hypertension

Think:

  • Malignancy
  • TB
  • Pancreatitis
  • Nephrotic syndrome
  • Infection

These patients need further workup.

Step 4 – Total protein (extra clue)

Helps refine cause.

Low protein (<25 g/L)

→ typical cirrhosis

High protein (>25 g/L)

→ think:

  • Malignancy
  • TB
  • Cardiac ascites
  • Pancreatitis

Not diagnostic alone — supportive only.

Step 5 – Culture results

Always send fluid directly into blood culture bottles (higher yield).

Positive culture

→ confirms infection

BUT remember:

SBP is diagnosed by neutrophils, not culture

Culture can be negative in true SBP.

Never delay treatment waiting for culture.

Step 6 – Special tests (only if indicated)

Order only when you have a reason.

Cytology

→ suspected malignancy

Amylase

→ suspected pancreatic ascites

Triglycerides

→ milky fluid (chylous)

AFB/TB culture

→ TB suspicion

Don’t send everything routinely — be targeted.

✅ Quick practical interpretation table (ward thinking)

Scenario 1

Neutrophils 900
→ SBP → antibiotics now

Scenario 2

SAAG 18
Protein low
→ Cirrhosis/portal hypertension

Scenario 3

SAAG 5
Protein high
→ Malignancy/TB → investigate further

Scenario 4

Bloody fluid + weight loss
→ think malignancy

Scenario 5

Milky fluid
→ think chylous ascites

Common real-life mistakes juniors make

  • Forgetting to check neutrophils first
  • Waiting for culture before treating
  • Not calculating SAAG
  • Sending wrong bottles
  • Over-ordering unnecessary tests
  • Ignoring clinical context

Remember:
Ascitic fluid supports your diagnosis — it doesn’t replace clinical judgement.

Ward workflow (simple mental checklist)

When results come back:

  1. Neutrophils → SBP?
  2. SAAG → portal hypertension or not?
  3. Protein → supportive clue
  4. Special tests → only if indicated

That’s it.

Keep it simple.

✅ Take-home concept

In ascitic fluid, neutrophils save lives and SAAG explains the cause.
Always rule out SBP first, then think about aetiology.