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CSF glucose, IgG and infection limits

18 min

Skills you'll build4 skills
  • Recognize why normal CSF findings cannot exclude a device-associated infection
  • Compare CSF glucose with a blood glucose drawn close to the lumbar puncture
  • Calculate the IgG index from paired CSF and serum IgG and albumin results
  • Distinguish CSF-restricted IgG bands from a matched systemic pattern

At the bench: Coordinate cell examination and chemistry for a body fluid specimen

Try first

Try first

An adult's lumbar CSF glucose is 60 mg/dL. A plasma glucose drawn at the time of the lumbar puncture is 240 mg/dL. How does the CSF glucose compare?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

CSF glucose and blood glucose

Glucose reaches the CSF from the blood, so CSF glucose rises and falls with blood glucose. It is commonly about 60% of a blood glucose drawn at about the same time.1 Every CSF glucose is interpreted with a blood glucose drawn close to the lumbar puncture. A low CSF glucose or a low CSF-to-blood ratio supports bacterial, tuberculous or fungal meningitis. In viral meningitis the glucose is often within the reference interval.2

The IgG index

IgG index = (CSF IgG ÷ serum IgG) ÷ (CSF albumin ÷ serum albumin). Match concentration units within each analyte pair. The index is dimensionless. Serum albumin is often reported in g/dL and CSF albumin in mg/dL, so one of them is converted before the albumin ratio is taken. The index is interpreted with the performing laboratory's interval. An increased index supports IgG production inside the central nervous system and does not identify a single disease.3,4

Oligoclonal bands need the serum lane

Isoelectric focusing separates the IgG in paired CSF and serum, and the two lanes are compared.4

PatternInterpretation
Bands in CSF with none at those positions in serumIgG made inside the central nervous system
The same bands at the same positions in both lanesMatched bands, IgG that came from the blood in a systemic immune response
Matched bands plus extra bands in CSF onlyA systemic response with IgG also made inside the central nervous system

Only CSF-restricted bands show intrathecal IgG production. Neither pattern identifies multiple sclerosis by itself, and band thresholds are set by each assay.4,5

Shunt infections

An infection around a ventricular shunt or drain can cause little inflammation. The cell count, glucose, protein and Gram stain can all be normal in an infected patient. Culture is the key test, and the report names the specimen source, such as a shunt reservoir or a drain.6

Follow the bench demonstration

Sources for this step
References
  1. Mayo Clinic Laboratories. Glucose, spinal fluid. Accessed September 27, 2026. https://www.mayocliniclabs.com/test-catalog/overview/152
  2. World Health Organization. WHO Guidelines on Meningitis Diagnosis, Treatment and Care. World Health Organization; 2025. Accessed September 27, 2026. https://www.who.int/publications/i/item/9789240108042
  3. Mayo Clinic Laboratories. Cerebrospinal fluid IgG index profile, serum and spinal fluid. Accessed September 27, 2026. https://www.mayocliniclabs.com/test-catalog/overview/610783
  4. Higgins V, Parker ML, Ahmed B, et al. Best practice recommendations for laboratory analysis and reporting of cerebrospinal fluid oligoclonal banding and associated tests for multiple sclerosis (MS): a consensus report from the harmonized CSF analysis for MS investigation (hCAMI) subcommittee of the Canadian Society of Clinical Chemists (CSCC). Clin Biochem. 2026;142:111098. doi:10.1016/j.clinbiochem.2026.111098
  5. Hoshina Y, Abbatemarco JR, Rodenbeck SJ, et al. Matched oligoclonal bands: diagnostic utility and clinical characteristics. Ann Clin Transl Neurol. 2024;11(11):2846-2854. doi:10.1002/acn3.52162
  6. Tunkel AR, Hasbun R, Bhimraj A, et al. 2017 Infectious Diseases Society of America's clinical practice guidelines for healthcare-associated ventriculitis and meningitis. Clin Infect Dis. 2017;64(6):e34-e65. doi:10.1093/cid/ciw861

Watch one

Read each decision and its explanation.

Paired CSF and serum are collected the same morning for an IgG index.

The laboratory's reference interval for the IgG index is 0.70 or less. What is the IgG index?

TestResultReference intervalFlag
CSF IgG6.0 mg/dL
Serum IgG1,000 mg/dL
CSF albumin30 mg/dL
Serum albumin4.0 g/dL

Specimen: Not measured on CSF. Lumbar CSF and serum, collected 40 minutes apart

  1. Check the pairing: CSF and serum were drawn 40 minutes apart.

    Serum IgG and albumin change over days, so the two specimens have to represent the same time.

  2. Check the units: both IgG results are in mg/dL, and serum albumin is in g/dL. Convert it: 4.0 g/dL = 4,000 mg/dL.

    Each ratio divides a CSF value by a serum value, and the units cancel only when they match.

  3. IgG ratio: 6.0 ÷ 1,000 = 0.0060.

    The IgG ratio shows how much IgG reached the CSF.

  4. Albumin ratio: 30 ÷ 4,000 = 0.0075.

    The central nervous system makes no albumin, so its ratio shows how much protein crossed from the blood.

  5. Index: 0.0060 ÷ 0.0075 = 0.80.

    Dividing by the albumin ratio corrects the IgG ratio for protein that leaked across the barrier.

  6. Compare: 0.80 is above the 0.70 limit, so the index is increased. Oligoclonal bands on the same pair are interpreted with it.

    The index is interpreted only with the performing laboratory's interval.

IgG index = (6.0 ÷ 1,000) ÷ (30 ÷ 4,000) = 0.80, increased against the laboratory's limit of 0.70.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A patient with a CSF shunt has suspected infection. What does a negative CSF Gram stain establish?

A negative Gram stain has limited sensitivity, particularly after antimicrobial exposure.

Normal chemistry and a negative Gram stain still do not exclude device-associated infection.

IDSA guidance retains culture and clinical correlation because a negative Gram stain cannot exclude device-associated infection.

Hint
  1. Consider how much inflammation a device infection causes.
  2. Consider which test can still detect organisms when the stain shows none.

Review CSF cell count and differential

Problem 2 of 3 · Hints available

Isoelectric focusing shows five IgG bands in the CSF lane. The paired serum lane shows five bands at the same five positions. What does the pattern show?

Every CSF band has a partner in serum. These bands came from the blood, so the CSF lane by itself overstates intrathecal IgG production.

Interpreted CSF bands without the paired serum pattern

Bands restricted to CSF support intrathecal IgG production, and bands at matching positions in CSF and serum reflect a systemic immune response. Reading CSF bands without the serum lane can report intrathecal synthesis from antibodies that came from the blood.

No band pattern identifies multiple sclerosis by itself. This matched pattern shows no IgG made inside the central nervous system.

The band pattern can be interpreted now. The IgG index is a separate calculation that adds its own information.

Bands at the same positions in both lanes reflect IgG that came from the blood. With no extra CSF bands, the pattern shows no intrathecal IgG production.

Hint
  1. Look at the serum lane before counting the CSF bands.
  2. Consider where IgG that appears at the same positions in both lanes was made.

Review CSF chemistry and immune studies

Problem 3 of 3 · On your own

Paired results are CSF IgG 4.5 mg/dL, serum IgG 1,100 mg/dL, CSF albumin 25 mg/dL and serum albumin 4.2 g/dL. What is the IgG index, to two decimal places?

Show the answer

0.69

Serum albumin = 4.2 g/dL = 4,200 mg/dL. IgG index = (4.5 ÷ 1,100) ÷ (25 ÷ 4,200) = 0.00409 ÷ 0.00595 = 0.69. Leaving serum albumin in g/dL gives 0.0007, because the albumin ratio is then 1,000 times too large.

Review CSF chemistry and immune studies

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Dmitri Spindler, 44, MRN 5520871, has a ventricular shunt and a new fever.
  • CSF is drawn from the shunt reservoir at 14:10, and a plasma glucose at 14:15.
  • The Gram stain shows no organisms. Culture is set up.
TestResultReference intervalFlag
CSF nucleated cells3 /µL
CSF glucose58 mg/dL
Plasma glucose98 mg/dL
CSF protein30 mg/dL

Specimen: Not measured on CSF. CSF from a ventricular shunt reservoir; clear and colorless

Decision 1 of 3

How does the CSF glucose compare?

CSF glucose is normally below the blood glucose, at about 60% of it.

58 ÷ 98 = 0.59, close to the usual 60%. The plasma was drawn 5 minutes after the CSF, so the two represent the same time.

A CSF glucose is interpreted with a blood glucose drawn close to it. A later fasting value represents a different time.

Review CSF chemistry and immune studies

Decision 2 of 3

How are the results reported?

A shunt infection can leave the count, glucose, protein and Gram stain normal. That comment could stop a workup that only the culture can complete.

Excluded a shunt infection from normal routine CSF findings

Device-associated ventriculitis and shunt infection can produce little inflammation, so a normal cell count, glucose, protein, and Gram stain can accompany infection. Reporting the specimen as showing no infection can end a workup that only culture would resolve.

The results are accurate, and the source shows how to interpret them. The culture is the test that can detect an infection.

The count, chemistry and stain are accurate now, and waiting days for the culture delays them without reason.

Review CSF cell count and differential

Decision 3 of 3

The nurse calls and asks whether the culture can be canceled, since everything else is normal. What do you tell her?

Normal routine findings are common with shunt infection. Canceling would remove the only test that can detect it.

Excluded a shunt infection from normal routine CSF findings

Device-associated ventriculitis and shunt infection can produce little inflammation, so a normal cell count, glucose, protein, and Gram stain can accompany infection. Reporting the specimen as showing no infection can end a workup that only culture would resolve.

The laboratory states the limitation of its results. Canceling an order is the decision of the clinician who placed it.

The Gram stain misses many infections, especially after antibiotics. After a negative stain, culture is the test that can still detect organisms.

Review CSF cell count and differential

The interpretation depends on the specimen source. CSF from a shunt can be entirely normal in an infected patient, so the paired glucose and the count are released as accurate results with the source named, and the culture continues.

For reference

Sources checked