Fluid Analysis Guidelines

Written by Craig Webster on 04 December 2025.

Below are some guidelines for fluid analyses which may be of clinical value. Please contact the Duty Biochemist if more information is required.

Fluid type	Clinical Indication	Analyses available	Sample Container	Comments

Ascitic Fluid	? cirrhotic or malignant	Albumin Cholesterol Triglycerides LDH	Plain universal	Serum albumin should be requested simultaneously measured for comparison.
	? chylous ascites	Chylomicrons Cholesterol Triglycerides	Plain universal
	? SBP (spontaneous bacterial peritonitis)	Total protein pH	Plain universal See comment	pH cannot be performed in a plain universal tube. Sample must be collected anaerobically into a lithium heparin tube and analysed on a blood gas analyser using a clot filter.
	? pancreatic fistula ?pancreatitis	Amylase	Plain universal	Serum amylase should be measured simultaneously for comparison. A high ascitic:serum amylase ratio is suggestive of a pancreatic origin.
	? tubercular ascites/ tuberculous peritonitis	Glucose	Fluoride oxalate (grey top)

Chest Drain Fluid	? chylothorax	Chylomicrons Cholesterol Triglyceride	Plain universal

CSF	? bacterial meningitis	Total protein Glucose Lactate	Plain universal Fluoride oxalate (grey top)
	? Subarachnoid haemorrhage	CSF bilirubin and oxyhaemoglobin (Xanthochromia) Total protein	Plain universal (protect sample from light)	Do not use pod system to send sample to lab. Samples must be received protected from light as light will cause degradation of bilirubin in the sample. Serum total protein and bilirubin should be measured simultaneously to support interpretation in equivocal cases.
	?congenital disorder ?cerebral ischaemia	Lactate	Fluoride oxalate (Grey top)	Raised levels of CSF lactate may occur with severe cerebral hypoxia or genetic lactic acidosis, intracranial haemorrhage, bacterial meningitis and epilepsy.
	? brain metastases	AFP, HCG, LDH	Plain universal	These analyses are not performed in-house and will be referred to Charing Cross Hospital. Turnaround may be up to 7 days.
	? neurosarcoidosis/ monitoring	ACE	Plain universal	These analyses are not performed in-house and will be referred to Queen’s Square Neurometabolic Laboratory. Turnaround may be up to 10 days. Please note, this test has not been clinically validated for diagnosis and monitoring of neurosarcoidosis and is therefore not UKAS accredited. This utility of this test should therefore be considered before requesting. Activity may be increased if blood-brain barrier impairment leads to passage of serum proteins (including serum ACE) or blood contamination.
	?Alzheimer’s Disease ?neurodegeneration	Beta-2-transferrin Amyloid Beta₄₂ and Amyloid Beta₄₀ ratio Total Tau Phospho-Tau¹⁸¹ S100B Neurofilament light chain	Polypropylene tube	These analyses are not performed in-house and will be referred to Queen’s Square Neuroimmunology Laboratory. Turnaround may be up to 15 days. The Duty Biochemist must be contacted via This email address is being protected from spambots. You need JavaScript enabled to view it. at least 1 day prior to sample collection. Polypropylene tubes must be collected from the Duty Biochemist in advance. Each individual test requires at least 500 µL sample, therefore discussion regarding sample volume needed is essential.
	Diagnosis and monitoring of inherited neurotransmitter disorders	CSF Neurotransmitters	Specialist neurotransmitter tubes	These analyses are not performed in-house and will be referred to Queen’s Square Neurometabolic Laboratory. Turnaround may be up to 35 days. Please contact Duty Biochemist (This email address is being protected from spambots. You need JavaScript enabled to view it.) at least 1 week prior to the sample being collected. Specialist tubes must be ordered from the referral lab and samples must be frozen on dry ice at the bedside. Requests must be accompanied by a specific referral lab form (in addition to the UHB request form) which be requested along with the tubes. A paired serum prolactin level may be useful. Patients should ideally be off any L-DOPA medications for 7 days prior to CSF collection.
	?narcolepsy with cataplexy	Orexin/Hypocretin	Plain universal	These analyses are not performed in-house and will be referred to Oxford for analysis. Turnaround may be up to 42 days.

Nasal/Ear Fluid	? CSF leak (rhinorrhoea/ otorrhoea)	Tau protein (Beta-2-transferrin)	Plain universal	These analyses are not performed in-house and will be referred to Sheffield protein reference unit. Turnaround may be up to 10 days.

Cyst Fluid	?thyroid tissue/ metastatic thyroid cancer	Thyroglobulin	Plain universal	Requires discussion with Duty Biochemist. Please email This email address is being protected from spambots. You need JavaScript enabled to view it. or phone 0121 371 6543.

Drain Fluid	? contains urine	Urea Creatinine	Plain universal	Comparison of fluid urea and creatinine with serum will identify significant contamination with urine
Drain Fluid	?post operative pancreatic fistula	Amylase Bilirubin	Plain universal	A high amylase and bilirubin in drained fluid suggests a leak of pancreatic fluid e.g. after pancreatic resection.

Gastric Aspirate	? reflux ?achlorhydria	pH	Plain universal	Occasionally gastric pH may be requested in patients suspected of intestinal reflux or achlorhydria. Normally the fasting gastric pH is about 1-2.

Pancreatic Cyst Fluid	Differentiation of pancreatic cysts types	CEA CA 19-9 Amylase Glucose	Plain universal Fluoride oxalate (grey top)	Pancreatic CEA may be useful in aiding differentiation between benign cysts and mucinous pancreatic cysts with an increased risk of malignancy. Please note, tumour markers are non-specific and should be used in conjunction with cytology and imaging.

Pleural Fluid Four types of fluids can accumulate in the pleural space: Serous fluid (hydrothorax) Blood (haemothorax) Chyle (chylothorax) Pus (pyothorax or empyema)	? transudate or exudates	Total Protein LDH (measure serum protein and LDH simultaneously)	Plain universal	A transudate fluid is produced through pressure filtration without capillary injury while exudate is "inflammatory fluid" leaking between cells. Most common causes of pleural exudates are bacterial pneumonia and malignancy. Most common causes of pleural transudates are left ventricular failure and cirrhosis. TP <25g/L indicates transudate TP>35g/L indicates exudate Light’s criteria applies to pleural fluid TP between 25 and 35g/L. A fluid is an exudate if any of the following apply: Ratio of fluid protein to serum protein is >0.5 Ratio of fluid LDH to serum LDH is >0.6 Pleural fluid LDH is > 2/3rds the upper reference limit for plasma LDH.
	? infected	pH	Lithium heparin (green top)	This is part of British Thoracic Society’s guidelines for differentiating infective from non-infective pleural effusions. pH can only be measured on a fresh specimen collected anaerobically using a dedicated blood gas analyser. At QE, this analyser can be found on W513 (respiratory). Suggest contact the POCT team to discuss prior to sample collection.
	? chylothorax	Chylomicrons Cholesterol Triglyceride	Plain universal	Visual inspection for chylomicrons required.
	?pancreaticopleural fistula/oesophageal rupture/malignancy	Amylase	Plain universal	Serum amylase should be measured for comparison. A level that is higher in pleural fluid compared to serum will aid differential diagnosis.
	?parapneumonic effusion/ tuberculosis/malignancy/ rheumatoid arthritis	Glucose	Fluoride oxalate (grey top)	Pleural fluid glucose is not solely diagnostic but may be useful in aiding differential diagnoses. A low pleural fluid glucose may suggest exudative effusions such as infections, tuberculosis, rheumatoid arthritis or malignancy.

Saliva	?Cushing’s syndrome/ Overnight dexamethasone suppression test	Salivary Cortisol	Sarstedt salivette (preferred)/ Passive drool into plain universal	Saliva specimens should be collected using a Sarstedt cortisol salivette (these can be requested from the Specialist testing laboratory by contacting the Duty Biochemist). Saliva collected into a plain container by passive drool is also acceptable. Recommend collection of late night salivary cortisol on two different occasions to aid initial diagnosis of Cushing’s syndrome.

Synovial Fluid	N/A	N/A		Refer to Microbiology

Urine pH	? cause of metabolic acidosis	pH (UPH)		In patients with a metabolic acidosis and suspected renal tubular acidosis, urine pH measurement is indicated. Please note, the reliability of a single spot urine pH measurement is limited. An early morning sample or testing after an acid load is more accurate. Urinary tract infections may also mimic distal RTA by causing a high urine pH. Measurement of serum bicarbonate alongside urine pH is most useful for interpretation.

biochemistry, blood sciences, clinical advice

Created on 28 February 2014.
Last updated on 04 December 2025.