Laboratory testing

1 Acid–Base disorders

For further discussion, see:

2 Anemia

for further discussion, see:

3 Anion gap

For further discussion, see:
This could be a more comprehensive list, but these are the commonest entities for the medical student

4 Cerebrospinal fluid (CSF)

Disease processGlucoseCellsProtein
TuberculosisLymphocytes, but mixed
Multiple sclerosisNormalNoneNormal (monoclonal bands)

5 Effusions

Gross appearanceAbsolute: < 2 gm/dLAbsolute: > 3 gm/dL
Protein content2Relative: < 50% plasmaRelative: > 50% plasma
Cells< 1000 /mL> 1000 /mL
• Glucose3≈ plasma glucose< plasma glucose
• LDH3< 50% plasmaAbsolute: > 2/3 upper limit of reference range4 || Relative: > 60% plasma
• pH3> 7.6< 7.3
1. Opaque: Red = blood, white = pus, foul smell = anaerobes
2. LDH levels > 1000 U/L represent pus (i.e. empyema)
3. In ascites, use serum albumin – ascites fluid albumin gap (SAAG): >1.1 = cirrhosis

6 Hypercalcemia

Bone pain

Kidney stones

GI complaints (pain, constipation, indigestion…)

Mental changes
    • Note that Vitamin D stores are best evaluated by a serum 25-hydoxyvitamin D (calcidiol). The active form of Vitamin D, 1,25-dihydoxyvitamin D (calcitriol), is made from calcidiol by the kidney enzyme 1α-hydroxylase, which is activated by PTH. Thus, calcitriol reflects PTH activity, not vitamin D stores and may actually be elevated in Vitamin D deficiency that has hypocalcemia with increased PTH.
    • Sarcoidosis will have an elevated 1,24 dihydroxyvitamin D as the macrophages have 1α-hydroxylase and the elevated Vitamin D is calcitriol.

7 Hyperglycemia acute

  • Polyuria, polydipsia & polyphagia (weight loss)
    • Diabetic ketoacidosis (DKA): abdominal pain & hyperventilation
    • Hyperosmolar, hyperglycemic syndrome (HHS): mental changes when serum osmolality > 325 mOsm/L (normal 285-295 mOsm/L)
Osmserum = 2(Na+) + Glu/18 + BUN/ 2.8
  • Differential diagnosis of hyperglycemia
    • Diabetes mellitus Type 1: Type IV hypersensitivity
    • Diabetes mellitus Type 2: insulin resistance followed over years by failure of β Cells
    • Hereditary defects of the β cell
    • Endocrine: a high level of a counter-regulatory hormone, either endogenous or therapeutic (i.e. cortisol)
      • Cortisol (e.g. Cushing disease)
      • Catecholamines (e.g. pheochromocytoma)
      • Growth hormone (e.g. acromegaly)
      • Glucagon (e.g. neuroendocrine tumor)
    • Pancreatic failure (e.g. cystic fibrosis, chronic pancreatitis)
    • Pregnancy: placental-derived insulin antagonists (e.g. human placental lactogen)

8 Hypoglycemia

For further discussion, see:
This is an uncommon abnormality and should only be investigated when Whipple’s triad is present:
  • Symptoms typical of hypoglycemia
    • Adrenal sympathetic: tremor, anxiety, palpitations, sweating, hunger…
    • Neurohypoglycemic: dizziness, drowsiness, mental changes…
  • Low blood sugar during symptoms
  • Symptoms relieved by glucose administration
Laboratory testing must be performed while symptoms are present.

9 Hyponatremia

10 Synovial fluid

Synovial fluid (for further discussion, see:

DiseasePathophysiologyJointPresentationSynovial fluid
DJD1MechanicalHeavy wearPain on motionNormal
RA2٠IgM anti-Fc IgG
٠Anti-CCP3 Ab
٠Morning stiffness
GoutUric acid crystalsPodagraNeedle-like painUrate crystals
CPPD5CPP crystalsKneesArthralgiaCPP crystals
SepticBacteria٠Polyarthritis (GC)
٠Monoarthritis (non-GC6)
AS7HLA-B27Vertebral bodySacroiliitisN/A
1. Degenerative joint disease, 2. Rheumatoid arthritis, 3. Anti-cyclic citrulline protein antibodies, 4. Systemic lupus erythematosus, 5. Calcium pyrophosphate crystal disease, 6, Gonococcus, 7. Ankylosing spondylitis

11 Urinalysis

Discussed in: