Fluid aspirates

Lumbar puncture procedure
– adults

• Withold if coagulopathy, thrombocytopaenia (plt <50, INR >1.5)
• In presence of aspirin, antiplatelets and NSAID’s must weigh risk/benefit as risks are not known
• Sterilisation of CSF occurs within 2 hours of parenteral antibiotics for meningococcal disease and 6 hours for pneumococcal disease
• Rapid latex agglutination tests are available for S. pneumonia, GBS, H. influenzae, E. coli and N. meningitidis but are associated with false positive and false negative results with limited sensitivity and specificity
• PCR is available for S. pneumoniae, N.meningitidis, GBS, H. influenzae, Listeria monocytogenes and M. tuberculosis but does not provide information on antibiotic susceptibility
• >20G needle doubles the risk of post-procedure headache
• Atraumatic or pencil-point needle (Whitacre or Sprotte) is associated with fewer post-procedural headaches as compared to Quinke cutting needles
• Smaller needle size using a stylet is associated with reduced headache

Head CT before LP?

• Altered mental status or deteriorating LOC
• Focal neurological deficit
• New onset seizure
• Papilloedema
• Immunocompromised
• Malignancy
• History of focal CNS disease
• Concern for CNS mass lesion
• Age >60yo

Csf pressure

• Normal pressure 5-17cmH20
• Raised in bacterial meningitis, cerebral venous sinus thrombosis, pseudotumor cerebri (benign intracranial hypertension)

CSF subarachnoid

• Xanthochromia
  • Tested by visual inspection or spectrophotometry
    • Spectro has higher sensitivity but lower specificity (75%) with resultant unnecessary testing for false positives
  • Takes 12 hours to develop
• RBC
  • The count in the 3^rd or 4^th tubes is typically used
  • 10-15% of LP’s are traumatic using cut-offs of 400 and 1000 RBD’s respectively
  • 25% reduction between tubes 1 and 4 can occur in causes of confimed SAH
  • RBC <100 in final tube effectively rules out SAH, whereas RCC >10 000 in final tube increased odds of SAH by factor of 6

Joint aspirate

Joint aspirate

• Relative contraindications
  • Overlying cellulitis/impetigo
  • Coagulopathy
  • Haemarthrosis in haemophiliac patients/significant coagulopathy before factor replacement
  • Prosthetic joint
• Injection of local into the joint can inhibit bacterial growth and cause false negative
• 18 or 19G needle for large joints, 20G for smaller joints

Septic arthritis

• Positive Gram stain in <50% of cases
• WCC >50000 is 56% sensitive and 90% specific
• WCC >25000 is 73% sensitive and 77% specific
• If crystals, WCC >2000 or >90% PMN suspect dual septic/gout
• If immunocompromised, WCC >200 and >25% PMN warrants consideration of IV AB’s and at least a period of observation
• Gonococcal arthritis has positive culture in <50%. Typically WCC 10 000 – 80 000

Periprosthetic infection

• 3 of the following (AAOS)
  • CRP >100 and ESR elevation
  • Synovial WCC >10 000
  • PMN >90%
  • Positive culture
  • Positive histological analysis of periprosthetic tissue

Shoulder joint aspirate

• Anterior approach
  • Betwee coracoid process and humeral head directed posteriorly
• Posterior approach
  • 1cm medial and 1cm inferior to the posterolateral corner of the acromion
  • Direct needle anteromedially toward coracoid process
  • Glenohumeral joint is 1-1.5 inches deep

Elbow aspiration

• Flex elbow to 90 degrees, resting forearm on table with hand prone
• Enter through centre of anconeus triangle (radial head, lateral epicondyle of humerus and lateral aspect of olecranon tip)
• Direct needle medially and perpendicular to radius towards distal end of antecubital fossa

Wrist aspiration

• Wrist prone
• Insert needle perpendicular to skin, ulnar to the radial tubercle and anatomical snuffbox, between extensor pollicus longus and common extensor tendons
• Wrist ulnar deviation widens the joint space

Knee aspiration

• Fully extend knee and relax quadriceps
• Insert lateral or medial to patella midpoint, 1cm inferior to patella edge
• Direct needle posterior to patella and horizontal toward joint space
• Can milk suprapatellar pouch
• Can also aspirate suprapatella pouch under USS guidance to one side of quadriceps tendon

Ankle aspirate

• Lateral (subtalar) approach
  • Just below tip of lateral malleolus directing needle medially
• Medial (tibiotalar) approach)
  • Sulcus lateral to medial malleolus and medial to tibialis anterior and extensor hallucis longus tendons
  • Plantar flex the foot and angle needle slightly cephalad as it passes between medial malleolus and tibialis anterior tendon

Peritoneal aspirate

• WCC >1000/mm3 or neutrophils > 250/mm3 suggests SBP
• Low glucose or high protein also suggests SBP
• Gram stain and culture can be falsely negative in 30-40%
• Culture sensitivity increased by using 10mL of ascitic fluid per blood culture bottle
• Can also test cytology, albumin, LDH and tumor markers

Pleural fluid

• Lights’ criteria (transudate vs. exudate)
  • Sensitivity for detection of exudative effusion is 98-99%
  • Specificity 65-86%
  • If thought to be exudative, further testing required
  • Diuretic therapy in transudative states can cause water reabsorption faster than protein, reaching exudative criteria
    • Serum:pleura albumin ratio >1.2g/dL has been proposed to confirm exudative but this reduces sensitivity of exudative pleural effusion detection by >10%
• Actual criteria are one or more of:
  • Pleural fluid:serum protein ratio >0.5
  • Pleural fluid: serum LDH ratio >0.6
  • Pleural fluid LDH >2/3 ULN for serum LDH

Pleural fluid

• If exudative by Lights criteria, need to also test:
  • Gram stain and culture
  • Cell count
    • Neutrophil predominance: Parapneumonic, PE, pancreatitis
    • Lymphocyte predominance: Cancer, TB, post-cardiac surgery
  • Glucose
    • Low glucose in parapneumonic, malignancy, TB and RA
  • Cytology for malignancy: Highest yield for adenocarcinoma (much lower for rest)
  • pH
    • In parapneumonic effusion: <7.10 predicts empyema formation or persistence and indicates drainage
  • Amylase: Elevated in pancreatitis/oesophageal rupture effusion
  • Mycobacterial and fungal stains and cultures as indicated
  • TB pleural fluid markers: PCR for M. tuberculosis, adenosine deaminase, IFN-gamma

Last Updated on October 2, 2020 by Andrew Crofton