Make me memorise how to interpret RBS

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Here is a concise, memorable guide to interpreting RBS (Random Blood Sugar / Random Blood Glucose) - taken directly from ADA/WHO criteria as cited in Robbins Pathology, Harriet Lane, and Goodman & Gilman.

How to Interpret RBS

The One Number You Must Know: 200 mg/dL (11.1 mmol/L)

Random means taken at ANY time of day, regardless of last meal.

The Simple Decision Tree

RBS result
    │
    ├─ < 140 mg/dL  (< 7.8 mmol/L)
    │       └─► NORMAL
    │
    ├─ 140 - 199 mg/dL  (7.8 - 11.0 mmol/L)
    │       └─► INDETERMINATE - cannot diagnose from RBS alone
    │           → Order FPG or OGTT for proper classification
    │
    └─ ≥ 200 mg/dL  (≥ 11.1 mmol/L)
            │
            ├─ WITH symptoms (polyuria, polydipsia, weight loss)?
            │       └─► DIABETES - confirmed, no repeat needed
            │
            └─ WITHOUT symptoms?
                    └─► DIABETES suspected - must REPEAT on a separate day
                        (one of: RBS ≥200, FPG ≥126, OGTT 2hr ≥200, HbA1c ≥6.5%)

Memory Anchor

"The magic number is 200." Random ≥ 200 + symptoms = Diabetes, done. Random ≥ 200 without symptoms = Diabetes, but confirm it.

All Diagnostic Cutoffs Side-by-Side

Test	Normal	Pre-diabetes	Diabetes
RBS (Random)	< 140 mg/dL	- (not used for pre-diabetes)	≥ 200 mg/dL
FPG (Fasting)	< 100 mg/dL	100-125 mg/dL	≥ 126 mg/dL
OGTT 2-hr	< 140 mg/dL	140-199 mg/dL	≥ 200 mg/dL
HbA1c	< 5.7%	5.7-6.4%	≥ 6.5%

Note: RBS is not used to diagnose pre-diabetes - there are no established pre-diabetes cutoffs for a random glucose value.

Key Exam Points to Remember

RBS does NOT require fasting - that is the whole point of it being "random."
Symptoms matter - with classic symptoms (polyuria, polydipsia, unexplained weight loss), a single RBS ≥200 is enough to diagnose diabetes. Without symptoms, you need confirmation on another day.
Acute stress hyperglycaemia is a trap - severe infections, burns, trauma can push glucose ≥200 transiently via catecholamines and cortisol. This is NOT diabetes. Wait for the acute illness to resolve before diagnosing.
RBS alone cannot diagnose pre-diabetes - for that, you need FPG, OGTT, or HbA1c.
All other tests need repeating - only RBS ≥200 with symptoms is a one-shot diagnosis.

Mnemonic: "200 SYMPTOMS = DONE"

2 = 200 mg/dL threshold
0 = no fasting required (zero prep)
0 = zero need to repeat IF symptoms are present
SYMPTOMS = polyuria, polydipsia, weight loss, candidal infections

Sources: Robbins & Kumar Basic Pathology; Harriet Lane Handbook (23rd ed.); Goodman & Gilman's Pharmacological Basis of Therapeutics; Park's Preventive & Social Medicine (WHO 2019 criteria)

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