44 Concise Book of Medical Laboratory Technology: Methods and Interpretations Conversion factors Analyte Conventional units SI units Conventional to SI units SI to conventional units Angiotensin-converting enzyme (ACE) nmol/min/mL U/L 1 1 Antidiuretic hormone (ADH) pg/mL ng/L 1 1 (vasopressin)

 

The SI system is based on meter-kilogram-second

system and replaces both the foot-pound-second system

and the centimeter-gram-second system. There are seven

SI base units, i.e. meter, kilogram, second, mole, ampere,

Kelvin and Candela. The symbols for these units and what

they measure are listed in Table 3.1.

LITER

The SI unit of volume is cubic meter (m3

). This is a very

large unit, hence, the liter (L) although not an SI unit, has

been recommended for use in the laboratory.

The liter is equal to a cubic decimeter (1 dm3

). Volume

measurements are made in liters or multiples and submultiples of the liter, e.g. dL (10-1L), mL (10-3L), µL (10-6L).

One liter is, therefore, equivalent to 10 dL, 1000 mL or

1000 000 µL. One dL is equivalent to 100 mL, and 1 mL to

1000 µL.

GRAM

The kilogram (kg) is the SI unit for mass and the gram (g)

is the working unit.

Formerly, the gram (g) was written as gramme, or gm.

Mass measurements are made in grams or in multiples

and submultiples of the gram, e.g. mg (10-3g), µg (10-6 g),

ng (10-9g), pg (10-12g).

One g is, therefore, equivalent to 1000 mg, 1000 000 µg,

or 1000 000 000 ng. One mg is equivalent to 1000 µg.

SI unit Old unit

nm m µ

µ m µ (micron)

MOLE (MOL)

The mole (mol) is the SI unit for amount of substance and

measurements of the amounts of substances are made in

moles, or in mmol (10-3 mol), µmol (10-6 mol), or nmol

(10-9 mol).

One mol is, therefore, equivalent to 1000 mmol, 1000

000 µmol, or 1000 000 000 nmol. One mmol/L is equivalent

to 1000 µmol/L.

Earlier, the results of tests expressed in mmol/L or

µmol/L were expressed in mg/100 mL or µg/100 mL.

SI unit Old unit

dL 100 mL

mL or cm3 cc

µL lambda

nL —

pL µ µ L

42 Concise Book of Medical Laboratory Technology: Methods and Interpretations

The formula used to convert mg/100 mL to mmol/L is as

follows:

mg/100 mL × 10 mmol/L = molecular weight of substance

where the molecular weight of a substance cannot be

accurately determined (e.g. albumin), results are expressed

in g/L.

INTERNATIONAL UNIT (U)

This unit is used to express enzyme activity. An

International Unit of enzyme activity is that amount

of enzyme which under defined assay conditions will

catalyze the conversion of 1 µmol of substrate per minute.

Results are expressed in International Units per liter (U/L).

CONVERSION FACTORS BETWEEN CONVENTION- AL AND SYSTEM INTERNATIONAL UNITS (SIU)

This list is included to assist the reader to convert values

between conventional units and the newer SI units that

have been mandated by many journals. Only common

analytes are included (Tables 3.2 to 3.5).

SI unit Old unit

mol M

mmol mEq

µmol µM

nmol nM

TABLE 3.1: The symbols of units and what they measure

SI base units SI unit prefixes

Symbol Quantity measured Prefix Symbol Function Divide by

Meter m Length deci d 10-1 10

Kilogram kg Mass centi c 10-2 100

Second s Time milli m 10-3 1000

Mole mol Amount of substance micro µ 10-6 1000 000

Ampere A Electric current nano n 10-9 1000 000 000

Kelvin K Temperature pico p 10-12 1000 000 000 000

Candela cd Luminous intensity femto f 10-15 1000 000 000 000 000

SI derived units Multiply by

Square meter m2 Area deca da 101 10

Cubic meter m3 Volume hecto h 102 100

Meter per second m/s Speed kilo k 103 1000

Named SI derived units mega M 106 1000 000

Hertz Hz Frequency giga G 109 1000 000 000

Joule J Energy, quantity of heat tera T 1012 1000 000 000 000

Newton N Force peta P 1015 1000 000 000 000 000

Pascal Pa Pressure

Watt W Power

Volt V Electric potential difference

degree Celsius °C Celsius temperature

SI Units 43

TABLE 3.2: Hematology

Conversion factors

Analyte

Conventional

units

SI

units

Conventional

to SI units

SI to conventional

units

WBC count (leukocytes) (B) µL or/cu mm or/mm3 cells 109

/L 0.001 1000

(CSF) /cu mm or 106

/L 1 1

→ cu µL 106

/L 106 10-6

(SF) #/µL #/L 106 10-6

Platelet count 103

/cu mm 109

/L 1 1

Reticulocytes /cu mm 109

/L 0.001 1000

RBC count (erythrocytes) (B) 106

/µL or /cu mm 1012/L 1 1

(CSF) or/mm3

/cu mm 106

/L 1 1

Hematocrit [packed cell volume (PCV)] % Volume fraction 0.01 100

Mean corpuscular volume (MCV) µ3

 (cubic microns) fL 1 1

(volume index)

Mean corpuscular hemoglobin (MCH) pg (or µg) pg 1 1

(color index) pg fmol 0.06206 16.11

Mean corpuscular hemoglobin g/dL g/L 10 0.1

concentration (MCHC) g/dL mmol/L 0.6206 1.611

(saturation index)

Hemoglobin g/dL g/L 10 0.1

(whole blood) g/dL mmol/L 0.155 6.45

(plasma) mg/dL µmol/L 0.155 6.45

Fetal hemoglobin % mol/mol (may omit symbol) 0.01 100

Haptoglobin mg/dL mg/L 10 0.1

Fibrinogen mg/dL g/L 0.01 100

Conversion factors

Analyte Conventional units SI units Conventional to SI units SI to conventional units

Adrenocorticotropic hormone (ACTH) pg/mL

pg/mL

ng/L

pmol/L

1

0.2202

1

4.541

Aldosterone

 (S) ng/dL nmol/L 0.0277 36.1

 (U) mEq/24 h mmol/d 1 1

 (U) µg/24 h nmol/d 2.77 0.36

Angiotensin ng/dL

pg/mL

ng/L

ng/L

10

1

0.1

1

TABLE 3.3: Chemistry

Contd...

44 Concise Book of Medical Laboratory Technology: Methods and Interpretations Conversion factors

Analyte Conventional units SI units Conventional to SI units SI to conventional units

Angiotensin-converting enzyme (ACE) nmol/min/mL U/L 1 1

Antidiuretic hormone (ADH) pg/mL ng/L 1 1 (vasopressin)

Albumin

 (S) g/dL g/L 10 0.1

 (CSF, AF) mg/dL mg/L 10 0.1

Alpha antitrypsin mg/dL g/L 0.01 100

Alpha-fetoprotein (AFP) ng/mL µg/L 1 1

 (S) ng/dL ng/L 10 0.1

mg/dL g/L 0.01 100

mg/dL mg/L 10 0.1