blood cell physiology

Question 1

define reference range

Answer 1

a carefully defined range derived from healthy population; includes results from middle 95% of population

Question 2

define normal range

Answer 2

looser term, where laboratory have looked at a collection of data and decided on own range; includes results from middle 95% of population

Question 3

what can affect “normal”

Answer 3

age, gender, ethnic origin, phsyiological status, altitude, nutritional status, cigarette smoking and alcohol intake (affect white cell count, red cell count and Hb)

Question 4

what is a reference range derived from

Answer 4

carefully determined reference population e.g. at that altitude, non-smokers, don’t drink more than 2 units/day

Question 5

how is a reference range determined

Answer 5

samples collected from healthy volunteers with defined characteristics; analysed using instrument and techniques used for patient samples; data analysed by appropriate statistical technique

Question 6

appropriate statistical technique for data with normal distribution e.g. Hb

Answer 6

determine mean and standard deviation and taking mean +/- 2SD as 95% range

Question 7

3 caveats to reference and normal ranges

Answer 7

not all results outside reference range are abnormal (e.g. for [Hb] at >95% is healthy), and not all results within normal range are normal (individuals differ), results within 95% range determined from apparently healthy people may still be abd for your health (e.g. high cholesterol in Western populations)

Question 8

due to caveats, what may be more meaningful than 95% range

Answer 8

health-related range tailored to individual: ideally sick and well have no overlap on tests, but best can hope for in practice is minimal overlap

Question 9

full blood count (FBC) abbreviations: WBC

Answer 9

white blood cell count in a given volume of blood (× 10^9/l)

Question 10

full blood count (FBC) abbreviations: RBC

Answer 10

red blood cell count in a given volume of blood (× 10^12/l)

Question 11

full blood count (FBC) abbreviations: Hb

Answer 11

haemoglobin concentration (g/l)

Question 12

full blood count (FBC) abbreviations: Hct/PCV

Answer 12

haematocrit (l/l)/packed cell volume (% or l/l) - older name for Hct

Question 13

full blood count (FBC) abbreviations: MCV

Answer 13

mean cell volume (fl)

Question 14

full blood count (FBC) abbreviations: MCH

Answer 14

mean cell haemoglobin (pg)

Question 15

full blood count (FBC) abbreviations: MCHC

Answer 15

mean cell haemoglobin concentration (g/l)

Question 16

full blood count (FBC) abbreviations: platelet count

Answer 16

number of platelets in a given volume of blood (× 10^9/l)

Question 17

how were WBC, RBC and platelet count calculated vs now

Answer 17

initially counted visually, using a microscope and a diluted sample of blood; now counted in large automated instruments

Question 18

how are WBC, RBC and platelet count calculated now in large automated instruments

Answer 18

by enumerating electronic impulses generated when cells flow between a light source and a sensor or when cells flow through an electrical field

Question 19

how was Hb calculated vs now

Answer 19

initially measured in a spectrometer by converting Hb to a stable form (cyanmethaemoglobin) and measuring light absorption at a specific wave length; now measured by an automated spectrophotometer (same principle)

Question 20

how was Hct (PCV) measured, and how is it now

Answer 20

by centrifuging a sample and measuring how much of column is packed red cells (%); now measured on automated instrument without packing, hence Hct

Question 21

how was MCV (fl) mathematically calculated

Answer 21

PCV (l/l) x 1000 / RBC (x10^-12/l)

Question 22

how is MCV now determined

Answer 22

indirectly by light scattering or by interruption of an electrical field

Question 23

what correlates with MCV on a blood film

Answer 23

cell size, so large cell (macrocyte) and small cell (microcyte)

Question 24

how is MCH (pg) calculated

Answer 24

Hb (g/l) / RBC (x10^-12/l)

Question 25

how is MCHC (g/l) calculated

Answer 25

Hb (g/l) / Hct (l/l)

Question 26

MCH vs MCHC; MCHC in spherocytes

Answer 26

MCH is absolute amount of Hb in individual red cell; MCHC is concentration of Hb in red cell and related to shape of cell; when cells become spherocytic, MCHC willl increase as same amount of Hb in smaller space

Question 27

in microcytic and macrocytic anaemias, what does MCH tend to parallel

Answer 27

MCV, so as cell gets larger, MCH gets larger, and vice versa

Question 28

how is MCHC now measured

Answer 28

electronically, most accurately on basis of light scattering

Question 29

what does MCHC correlate with

Answer 29

hypochromia

Question 30

how is a blood count interpreted: white cells

Answer 30

is there leucocytosis or leucopenia, and if so why; which cell line is abnormal; any clues in clinical history

Question 31

how is a blood count interpreted: red cells

Answer 31

is there anaemia, and if so any clues in blood count; size of cells; any clues in clinical history

Question 32

how is a blood count interpreted: platelets

Answer 32

is there thrombocytosis or thrombocytopenia, and if so any clues in blood count; any clues in clinical history

Question 33

4 things to interpret initially in a blood count (most important)

Answer 33

WBC and differential (absolute count not %), Hb, MCV, platelet count

Question 34

besides blood test, what else might you require for a correct blood count interpretation

Answer 34

blood film (e.g. for sickle cell anaemia - high platelet count also as splenic dysfunction)