# Lumbar Puncture: Indications, Technique, and CSF Interpretation

## Introduction

Lumbar puncture (LP) is a fundamental diagnostic and therapeutic procedure in internal medicine. Mastery of its indications, contraindications, technique, and **cerebrospinal fluid (CSF) interpretation** is essential for the evaluation of meningitis, encephalitis, subarachnoid hemorrhage, and numerous neurological conditions.

## Indications

### Diagnostic

- **Suspected meningitis or encephalitis**: bacterial, viral, fungal, tuberculous
- **Subarachnoid hemorrhage (SAH)**: when CT head is negative but clinical suspicion remains high
- **Suspected CNS malignancy**: leptomeningeal carcinomatosis, CNS lymphoma
- **Demyelinating diseases**: multiple sclerosis (oligoclonal bands, IgG index)
- **Normal pressure hydrocephalus**: large-volume LP as both diagnostic and therapeutic trial
- **Guillain-Barre syndrome**: albuminocytologic dissociation (elevated protein, normal cell count)

### Therapeutic

- **Idiopathic intracranial hypertension (pseudotumor cerebri)**: reduce elevated opening pressure
- **Intrathecal medication administration**: chemotherapy, antibiotics

## Contraindications

- **Absolute**: skin infection at the puncture site, spinal epidural abscess at LP level
- **Relative**: elevated intracranial pressure with mass lesion (risk of herniation), severe coagulopathy (INR > 1.5, platelets < 50,000), anticoagulation therapy
- **CT head before LP** is indicated if: focal neurological deficits, papilledema, altered mental status, immunocompromised state, seizure within 1 week, or age > 60
- In suspected bacterial meningitis, **do not delay antibiotics** for imaging or LP

## Technique

### Patient Positioning

- **Lateral decubitus** (fetal position): preferred for accurate opening pressure measurement; knees to chest, chin to chest
- **Seated upright**: easier landmark identification in obese patients but opening pressure is unreliable
- Identify the **L3-L4 or L4-L5 interspace** using the intercristal line (Tuffier's line)

### Procedure Steps

1. Obtain informed consent and perform a timeout
2. Sterilize the area and apply sterile drape
3. Anesthetize the skin and subcutaneous tissue with **1% lidocaine**
4. Insert a **20-22 gauge atraumatic (Sprotte or Whitacre) needle** with the bevel parallel to the longitudinal dural fibers
5. Advance through skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, and dura
6. A **pop** or give is often felt upon entering the subarachnoid space
7. Measure **opening pressure** with the patient relaxed and legs extended
8. Collect CSF into **4 sequential tubes** (typically 1-2 mL per tube)
9. Replace the stylet before withdrawing the needle

![Lumbar puncture anatomical landmarks and needle trajectory](images/lp-anatomy-technique.png)

### Ultrasound Guidance

- POCUS can identify the midline, interspace, and depth to the posterior complex
- Particularly useful in patients with obesity or difficult landmarks
- Reduces number of attempts and improves success rate

## Opening Pressure

- **Normal**: 6-20 cm H2O in lateral decubitus
- **Elevated (> 25 cm H2O)**: bacterial or fungal meningitis, idiopathic intracranial hypertension, cerebral venous sinus thrombosis
- **Low (< 6 cm H2O)**: CSF leak, dehydration, post-LP state
- Ensure patient is relaxed with legs extended; Valsalva or anxiety falsely elevates pressure

## CSF Interpretation

### Normal Values

- **WBC**: 0-5 cells/mcL (all mononuclear)
- **Protein**: 15-45 mg/dL
- **Glucose**: 50-80 mg/dL (or > 60% of serum glucose)
- **Appearance**: clear and colorless

### Patterns by Disease

| Condition | WBC | Predominant Cell | Protein | Glucose |
|-----------|-----|-------------------|---------|---------|
| **Bacterial meningitis** | 1,000-10,000+ | Neutrophils | Elevated (>100) | Low (<40) |
| **Viral meningitis** | 10-500 | Lymphocytes | Normal-mild elevation | Normal |
| **TB/Fungal meningitis** | 50-500 | Lymphocytes | Elevated (>100) | Low |
| **GBS** | Normal | -- | Elevated | Normal |
| **MS** | 0-50 | Lymphocytes | Normal-mild elevation | Normal |
| **SAH** | Elevated RBCs | RBCs | Elevated | Normal |

### Traumatic Tap vs. True SAH

- **Traumatic tap**: RBC count decreases from tube 1 to tube 4; CSF clears with sequential tubes
- **True SAH**: RBC count remains constant across tubes; **xanthochromia** (yellow supernatant) is present after centrifugation
- Xanthochromia develops **2-12 hours** after SAH onset and can persist for weeks

![CSF interpretation diagnostic algorithm](images/csf-interpretation-algorithm.png)

## Complications

- **Post-LP headache**: occurs in 10-30% of patients; positional (worse upright, better supine); use atraumatic needles to reduce risk
- **Epidural blood patch**: definitive treatment for persistent post-LP headache; 85-90% effective
- **Infection and bleeding**: rare with proper technique
- **Cerebral herniation**: prevented by appropriate pre-LP imaging when indicated

## Special Considerations

- **Anticoagulation**: hold warfarin until INR < 1.5; hold LMWH for 12 hours; hold DOACs for 24-72 hours depending on agent
- **Send appropriate studies**: cell count (tubes 1 and 4), protein, glucose, Gram stain, culture; additional studies as indicated (HSV PCR, cryptococcal antigen, cytology, oligoclonal bands)

![Post-lumbar puncture headache management algorithm](images/post-lp-headache.png)

## Key Clinical Pearls

- Never delay empiric antibiotics for LP or imaging in suspected bacterial meningitis.
- Use atraumatic (Sprotte/Whitacre) needles to significantly reduce post-LP headache rates.
- Opening pressure must be measured in the lateral decubitus position with legs extended for accuracy.
- Xanthochromia distinguishes true SAH from a traumatic tap and requires 2-12 hours to develop.
- Always send cell count on both tube 1 and tube 4 to help differentiate traumatic tap from true hemorrhage.

## References

1. Engelborghs S, Niemantsverdriet E, Struyfs H, et al. Consensus Guidelines for Lumbar Puncture in Patients with Neurological Diseases. *Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring*. 2017;8:111-126.
2. Nath S, Koziarz A, Badhiwala JH, et al. Atraumatic versus Conventional Lumbar Puncture Needles: A Systematic Review and Meta-analysis. *The Lancet*. 2018;391(10126):1197-1204.
3. Tunkel AR, Hartman BJ, Kaplan SL, et al. Practice Guidelines for the Management of Bacterial Meningitis. *Clinical Infectious Diseases*. 2004;39(9):1267-1284.
4. Doherty CM, Forbes RB. Diagnostic Lumbar Puncture. *Ulster Medical Journal*. 2014;83(2):93-102.