# Haemoglobin Estimation Methods > Haemoglobin estimation methods for Medical Laboratory Technology: Drabkin cyanmethaemoglobin, Sahli, WHO colour scale and SLS with anaemia cut-offs. - Canonical URL: https://prepelephant.com/topics/allied/medical-laboratory-technology/haemoglobin-estimation-methods - Exam / course: Allied Health · Subject: Medical Laboratory Technology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Haemoglobin Estimation Methods", PrepElephant, https://prepelephant.com/topics/allied/medical-laboratory-technology/haemoglobin-estimation-methods ## Direct answer Drabkin's cyanmethaemoglobin method has anchored haemoglobin measurement since the mid-twentieth century and remains the international reference: all haemoglobin derivatives are converted to stable cyanmethaemoglobin (HiCN) by potassium ferricyanide and potassium cyanide, read spectrophotometrically at 540 nanometres against a certified standard. On Indian benches, Sahli's acid-haematin method survives in teaching laboratories for its simplicity, the WHO haemoglobin colour scale serves field and antenatal screening, and automated analysers increasingly run the cyanide-free sodium lauryl sulphate (SLS) chemistry. WHO anaemia thresholds for adults: below 13 g/dL in men, below 12 g/dL in non-pregnant women and below 11 g/dL in pregnant women. ## What you must remember - Drabkin's principle: ferricyanide oxidises haemoglobin's iron to the ferric state (methaemoglobin), cyanide then binds it as stable cyanmethaemoglobin, read at 540 nm. - Sahli's method: 20 microlitres of blood into 0.1 N hydrochloric acid forms brown acid haematin, matched against the standard brown glass; simple but subjective. - WHO haemoglobin colour scale: eight shades spanning 4-14 g/dL, reliable to within about 1 g/dL in trained hands — the field tool for anaemia screening under national programmes. - SLS method: sodium lauryl sulphate forms a coloured complex without cyanide, making it the safer automated option; results agree closely with HiCN. - Adult references: men roughly 13-17 g/dL, women roughly 12-15 g/dL; pregnancy accepts down to 11; severe anaemia is generally below 7 g/dL. - The rule of three: haemoglobin × 3 approximates packed cell volume; a large deviation means measurement error, lipaemia or an abnormal cell population. - Drabkin's reagent contains cyanide: never acidify it (hydrogen cyanide gas) and discharge waste through the laboratory's chemical disposal route. - Turbidity from very high white cell counts, lipaemia or abnormal proteins falsely elevates HiCN readings — dilute and repeat, or clarify the sample. ## Running a Sahli's estimation honestly The Sahli (acid-haematin) haemometer is still a fixture of university practicals, and examiners grade the honesty of the technique. Hydrochloric acid (0.1 N) is drawn into the graduated tube up to the 2 g/dL mark. Exactly 20 microlitres of well-mixed EDTA blood is pipetted, the pipette tip wiped, and the blood blown into the acid and rinsed through two or three times by drawing and expelling the mixture. Then comes the step impatience destroys: waiting five to ten minutes, because acid haematin's colour develops slowly — students who dilute immediately read a falsely low value. After the wait, distilled water is added drop by drop, mixing after each drop by stirring with the glass rod, until the colour of the dilute mixture matches the standard brown glass blocks beside it. The reading is taken at the lower meniscus, at eye level, and the dilution volume directly gives haemoglobin in g/dL. Every step carries a mark: wiping the pipette (excess blood on the tip overestimates), waiting for colour development, dropwise rather than bulk dilution, and reading against good daylight. Compare the finished honesty of this visual matching with Drabkin's — a stable chromogen, a fixed wavelength, a traceable standard — and the reason HiCN became the reference is self-evident: it removed the human eye from the measurement. ## Where students slip Three slips recur in vivas. First, answering "why 540 nanometres?" with a shrug — it is the absorption maximum of cyanmethaemoglobin, the entire justification for the wavelength. Second, forgetting the disposal question: a reagent containing potassium cyanide is treated as hazardous chemical waste, never poured down a sink, and certainly never mixed with acid. Third, confusion of units — Indian laboratories report g/dL while journals report g/L (1 g/dL equals 10 g/L), and a candidate mixing them produces a tenfold error that no examiner forgives. ## Frequently asked questions ### Which haemoglobin method is the international reference and why? The cyanmethaemoglobin (Drabkin's) method — virtually all haemoglobin derivatives convert to one stable pigment read at 540 nm against a certified ICSH standard. ### What is the principle of the Sahli method? Blood is converted to brown acid haematin by 0.1 N HCl and visually matched against permanent glass standards after dropwise dilution. ### What are the WHO cut-offs for anaemia in adults? Below 13 g/dL in men, below 12 in non-pregnant women, below 11 in pregnant women; severe anaemia is generally below 7 g/dL. ### How does the WHO haemoglobin colour scale work? A drop of blood on filter paper is matched against eight standard shades from 4-14 g/dL, accurate to about 1 g/dL when used correctly. ### Why are laboratories moving away from cyanide methods? Cyanide toxicity and disposal hazard; cyanide-free SLS chemistry provides comparable results with safer waste. ### What causes false elevation of HiCN readings? Turbidity from marked leucocytosis, lipaemia, high abnormal protein levels or inadequate red cell lysis — resolve by dilution or centrifugation and repeat.