Central Nervous System Infections

InCheat Sheets / Infectious Disease / Internal Medicine

Updated on November 18, 2025

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Overview
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Overview

CNS infections carry a high mortality if untreated or improperly treated. It is important to recognize and treat differently neonates, young people, adults, old people, and immunocompromised since different organisms affect these different populations.

Infections of the CNS include:

Infection	Description
Meningitis	Infection of the meninges
Encephalitis	Generalized infection of the brain parenchyma
Brain abscess	Focal infection and suppuration of the brain parenchyma

Symptom triad of CNS infections
- Fever
- Headache
- Altered mental status

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Dr. Jeffrey Kalei

Author and illustrator for Hyperexcision. Interested in emergency room medicine. I have a passion for medical education and drawing.

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Biochemistry

Adrenocorticotrophic hormone	P: <80 ng/L
Alanine aminotransferase (ALT)	P: 5–35 U/L
Albumin	P: 35–50 g/L
Aldosterone	P: 100–500 pmol/L
Alkaline phosphatase	P: 30–130 U/L (adults)
α-Amylase	P: 0–180 IU/dL
α-Fetoprotein	S: <10 kU/L
Angiotensin II	P: 5–35 pmol/L
Antidiuretic hormone (ADH)	P: 0.9–4.6 pmol/L
Aspartate transaminase	P: 5–35 U/L
Bicarbonate	P: 24–30 mmol/L
Bilirubin	P: 3–17 μmol/L
BNP	P: <50 ng/L
C-reactive protein	P: <10 mg/L
Calcitonin	P: <0.1 mcg/L
Calcium (ionized)	P: 1.0–1.25 mmol/L
Calcium (total)	P: 2.12–2.60 mmol/L
Chloride	P: 95–105 mmol/L
Cholesterol	P: <5.0 mmol/L
VLDL	P: 0.128–0.645 mmol/L
LDL	P: <2.0 mmol/L
HDL	P: 0.9–1.93 mmol/L
Cortisol	P: AM 450–700 nmol/L, Midnight 80–280 nmol/L
Creatine kinase (CK)	P: ♂ 25–195 U/L, ♀ 25–170 U/L
Creatinine	P: 70–100 μmol/L
Ferritin	P: 12–200 mcg/L
Folate	S: 2.1 mcg/L
FSH	P/S: 2–8 U/L ♂ (luteal); >25 U/L menopause
Gamma-glutamyl transpeptidase	P: ♂ 11–51 U/L, ♀ 7–33 U/L
Glucose (fasting)	P: 3.5–5.5 mmol/L
Growth hormone	P: <20 mu/L
HbA1C (DCCT)	B: 4–6%
HbA1C (IFCC)	B: 20–42 mmol/mol
Iron	S: ♂ 14–31 μmol/L, ♀ 11–30 μmol/L
Lactate	P: Venous 0.6–2.4 mmol/L, Arterial 0.6–1.8 mmol/L
Lactate dehydrogenase (LDH)	P: 70–250 U/L
Lead	B: <1.8 mmol/L
LH	P: 3–16 U/L (luteal)
Magnesium	P: 0.75–1.05 mmol/L
Osmolality	P: 278–305 mosmol/kg
Parathyroid hormone (PTH)	P: 0.8–8.5 pmol/L
Potassium	P: 3.5–5.3 mmol/L
Prolactin	P: ♂ <450 U/L, ♀ <600 U/L
PSA	P: 0–4 mcg/mL
Protein (total)	P: 60–80 g/L
Red cell folate	B: 0.36–1.44 μmol/L
Renin	P: Erect 2.8–4.5, Recumbent 1.1–2.7 pmol/mL/h
Sodium	P: 135–145 mmol/L
TBG	P: 7–17 mg/L
TSH	P: 0.5–4.2 mU/L
Thyroxine (T4)	P: 70–140 nmol/L
Free T4	P: 9–22 pmol/L
Total iron-binding capacity	S: 54–75 μmol/L
Triglycerides (fasting)	P: 0.50–2.3 mmol/L
Triiodothyronine (T3)	P: 1.2–3.0 nmol/L
Troponin T	P: <0.1 mcg/L
Urate	P: ♂ 210–480 μmol/L, ♀ 150–390 μmol/L
Urea	P: 2.5–6.7 mmol/L
Vitamin B12	S: 0.13–0.68 nmol/L
Vitamin D	S: 50 nmol/L

Arterial Blood Gases

pH	7.35–7.45
PaCO₂	4.7–6.0 kPa
PaO₂	>10.6 kPa
Base excess	±2 mmol/L

Urine

Cortisol (free)	<280 nmol/24h
Hydroxyindole acetic acid	16–73 μmol/24h
Hydroxymethylmandelic acid	16–48 μmol/24h
Metanephrines	0.03–0.69 μmol/mmol creatinine
Osmolality	350–1000 mosmol/kg
17-Oxogenic steroids	♂ 28–30 μmol/24h, ♀ 21–66 μmol/24h
17-Oxosteroids (neutral)	♂ 17–76 μmol/24h, ♀ 14–59 μmol/24h
Phosphate (inorganic)	15–50 mmol/24h
Potassium	14–120 mmol/24h
Protein	<150 mg/24h
Protein/creatinine ratio	<3 mg/mmol
Sodium	100–250 mmol/24h

Haematology

WCC	4.0–11.0 ×10⁹/L
Red cell count	♂ 4.5–6.5 ×10¹²/L, ♀ 3.9–5.6 ×10¹²/L
Haemoglobin	♂ 130–180 g/L, ♀ 115–160 g/L
PCV / Haematocrit	♂ 0.4–0.54 L/L, ♀ 0.37–0.47 L/L
MCV	76–96 fL
MCH	27–32 pg
MCHC	300–360 g/L
RDW	11.6–14.6%
Neutrophils	2.0–7.5 ×10⁹/L (40–75%)
Lymphocytes	1.0–4.5 ×10⁹/L (20–45%)
Eosinophils	0.04–0.44 ×10⁹/L (1–6%)
Basophils	0–0.10 ×10⁹/L (0–1%)
Monocytes	0.2–0.8 ×10⁹/L (2–10%)
Platelet count	150–400 ×10⁹/L
Reticulocyte count	0.8–2.0% / 25–100 ×10⁹/L
ESR	Depends on age
Prothrombin time	10–14 s
APTT	35–45 s

Paediatric

Pulse Rate (beats per minute)
Neonate (<1 month)	140–160
Infant (<1 year)	120–140
1–5 years	110–130
5–12 years	80–120
>12 years	70–100
Temperature
Normal range	36.6–37.2 °C
38.0 °C	Fever (infant)
38.5 °C	Fever (>3 months)
Respiratory Rate (tachypnoea)
Neonate (0–2 months)	≥60 breaths/min
2–12 months	≥50 breaths/min
1–5 years	≥40 breaths/min
>5 years	≥30 breaths/min
Blood Pressure (mmHg)
Term	65/45
1 year	75/50
4 years	85/60
8 years	95/65
10 years	100/70
Head Circumference
Term	35 cm ± 2
1 year	+1 cm/month
2–3 years	+2 cm/year
4–5 years	+1 cm/year
5 years	~53 cm
10 years	56–57 cm
Height
Term	52–55 cm
1 year	75 cm
2–12 years	6a + 77 cm
Weight Formulas
3–12 months	(a + 9)/2 kg
1–6 years	2a + 8 kg
>6 years	(7a − 5)/2 kg
Developmental Milestones
Social smile	1.5 months
Head control	4 months
Sits unsupported	7 months
Crawls	10 months
Stands unsupported	10–12 months
Walks	12–13 months
Talks	18 months
MUAC (6 months–5 years)
Obese	>17.5 cm
Normal	13.5–17.4 cm
At risk	12.5–13.4 cm
Moderate malnutrition	11.5–12.4 cm
Severe malnutrition	<11.5 cm
Blood Transfusion
Whole blood	20 mL/kg
pRBC	10 mL/kg
Formula	6 × (weight in kg) × Hb deficit
Normal Paediatric Lab Values
Haemoglobin (g/dL)	Hematocrit (%)
Term newborn: 13–20	40–58
1 month: 11–18	32–54
2 months: 10–15	28–44
1–2 years: 10–13	32–40
>2 years: 11–14	33–42
CSF WBC (/mm³)
Term newborn	0–25
>2 weeks	0–5

Overview

Videos

Dr. Jeffrey Kalei

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