Lactate and glucose in abdominal fluid in cats and dogs

What it shows

These two codes are the familiar blood tests measured in fluid from the abdomen (sometimes from another body cavity):

• Effusion glucose — in a non-septic effusion glucose is close to the blood level. In septic peritonitis glucose in the fluid drops markedly.
• Effusion lactate — lactate is a product of energy use without enough oxygen. In a non-septic effusion it is lower in the fluid than in blood; in an infected or poorly supplied abdomen it can become higher in the fluid.

As with other fluid tests, the useful figure is not the fluid value alone but its difference from, or ratio to, the blood value measured at the same time.

Higher than reference — what it may be a sign of

Lactate in the fluid higher than in blood supports a suspicion of septic peritonitis. In a study of dogs, fluid lactate at least 2.0 mmol/L higher than blood lactate was 100% sensitive and specific for septic effusion, but this was measured in only seven dogs. Lactate in the fluid higher than in blood can also occur without infection — for example in dogs with gastric dilatation-volvulus — so a high lactate is not specific to infection.

A blood glucose higher than fluid glucose is the other side of the same picture — see the next section.

Lower than reference

Glucose in the fluid lower than in blood supports a suspicion of septic peritonitis:

• A fluid-to-blood glucose ratio below 0.5 is described as highly sensitive and specific, and in septic fluid glucose often falls below what the analyser can detect.
• In a study of 18 dogs, a blood glucose at least 20 mg/dL (about 1.1 mmol/L) higher than fluid glucose was 100% sensitive and specific for septic effusion; the difference was more accurate than fluid glucose alone.
• Later studies gave lower and differing figures: with a handheld glucose meter the same 20 mg/dL difference found fewer than half of septic effusions, and in a study on a blood gas analyser about a third of non-septic effusions were classed as septic. After abdominal surgery, glucose from a drain was not reliable.

A summary of the evidence in dogs calls glucose a useful bedside test that cannot replace cytology and culture.

Look at together with

• Fluid cytology — bacteria seen in the fluid, especially inside neutrophils, confirm septic peritonitis.
• Culture and susceptibility — identifies the bacteria; samples are taken for culture when septic peritonitis is suspected.
• Protein and cell count of the fluid — an infected fluid is an exudate with many neutrophils.
• Blood glucose and lactate — the paired values needed for the comparison.

Units and conversion

Effusion lactate is written in mmol/L. Effusion glucose is written in mmol/L or mg/dL: 1 mg/dL = 0.05551 mmol/L, so a difference of 20 mg/dL is about 1.1 mmol/L. The comparison only works when the fluid and the blood value are in the same unit and, ideally, measured on the same analyser. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form, but for fluid from a body cavity there is usually no “normal” interval. The differences and ratios above are guides from small studies, not a lab’s reference; the vet reads them together with the cells in the fluid, the culture and the animal’s condition.

Cats and dogs

Sources

1. Merck Veterinary Manual — Peritonitis in Animals
2. Bonczynski JJ et al. Peritoneal fluid versus peripheral blood glucose and lactate for the diagnosis of septic peritonitis in dogs and cats. Vet Surg. 2003 (doi 10.1053/jvet.2003.50005)
3. Veterinary Evidence (RCVS Knowledge) — Can measurement of glucose levels in blood and peritoneal fluid be used to diagnose septic peritonitis in dogs?
4. eClinPath (Cornell University) — Peritoneal fluid