Laboratory mathematics and measurement

All laboratory testing relies on the accurate use of values, units, and measurements. For example, values are used for reporting the time the sample was collected, the volume of the specimen, the amount of analyte found in a specimen, and dilutions used in sample preparation and for recording QC results.

Measuring time

Time of day is a critical factor in patient care. Medications must be administered, diets must be followed, and specimens must be collected on a timed schedule. Many laboratories use the 24-hour clock when recording time; this method avoids the confusion that comes with the 12-hour clock, which uses a.m. (morning) and p.m. (afternoon) designations.

The 24-hour clock system, also known as military time, is expressed with four digits in terms of “hundred hours.” Noon is referred to as 1200 (twelve hundred) hours; midnight is 0000 (zero hundred), or 2400 hours. Use 0000 to mark the start of a new day and 2400 to mark the end of the day that’s finishing - both notations refer to the same moment, midnight. The military clock is based on 24 60-minute hours, as is the 12-hour clock; therefore, 5:35 p.m. is expressed as 1735 (seventeen thirty-five) hours.

Measuring temperature

Two scales are currently used for measuring temperature; each is divided into units called degrees. The Fahrenheit scale is considered part of the English system of measurement and is the scale most commonly used in the United States. The Celsius scale, formerly called the centigrade scale, is used in countries that apply the metric system. On the Celsius © scale, water freezes at 0°C and boils at 100°C. On the Fahrenheit (F) scale, water freezes at 32°F and boils at 212°F. Almost all laboratories in the United States use the Celsius scale for temperature.

Common laboratory temperature settings

	Fahrenheit	Celsius
Refrigerator	35°-46°	2°-8°
Freezer	32°	0°
Room	59°-86°	15°-30°
Incubator / body temperature	98.6°	37°
Autoclave	250°	121°

Converting temperatures

To change from Fahrenheit to Celsius

$$98.6^{\circ }F={\underline{}}^{\circ }C$$

Step 1: Subtract 32 from the Fahrenheit temperature.

Step 2: Divide the result by 1.8.

To change from Celsius to Fahrenheit

$$100^{\circ }C={\underline{}}^{\circ }F$$

Step 1: Multiply the Celsius temperature by 1.8.

Step 2: Add 32 to the result.

Units of measurement

The units of measurement we commonly use in the United States differ from those used in the laboratory. In everyday life, we use the English system of measurement. In the laboratory, the metric system and the Système International (SI) are used. It is important for medical assistants to memorize and practice these systems so that they can communicate professionally.

The metric system is based on a decimal system, which consists of basic units and prefixes that indicate a system of division in multiples of 10. Prefixes are added to each symbol to reduce or enlarge them by units of 10. International organizations, such as the World Health Organization (WHO), officially recognize SI units. Many countries have adopted this system, but the United States has not completely converted to it. The SI is an adaptation of the metric system that uses several basic units, although some units are different for reporting results. For example, blood glucose is reported in millimoles per liter (mmol/L) using the SI system, but it is reported as milligrams per deciliter (mg/dL) using the metric system. Therefore, it is very important for the medical assistant to double-check the laboratory’s standard and include the appropriate units of measurement when reporting test values.

Metric prefixes: the metric system builds units by attaching a prefix to a base unit (like the liter or gram). Each prefix shifts the value by a power of ten:

• Kilo- (k): ×1,000
• Deci- (d): ×0.1 (1/10)
• Centi- ©: ×0.01 (1/100)
• Milli- (m): ×0.001 (1/1,000)
• Micro- (µ): ×0.000001 (1/1,000,000)

For example, to convert $2.5$ cL to microliters: first convert to liters ($2.5\times 0.01=0.025$ L), then convert liters to microliters ($0.025\times 1,000,000=25,000$ µL).

Comparing the English system of measurement with the metric system

Category	English system of measurement	Metric system
Weight	Ounces (oz) and pounds (lb)	Grams (g)
Length	Inches and feet	Meters (m)
Volume	Cups and quarts	Liters (L)

Measuring liquid volume

Test tubes are used to test or hold liquid reagents, samples, or aliquots. Test tubes come in many sizes and are typically disposable. Test tubes may be sterile for use in microbiology. When liquids are measured into test tubes, the most common piece of glassware used is the pipet. A pipet is a hollow tube that can be made from glass or plastic. Pipets often have lines to indicate volume on the length of the tube. Some plastic pipettes have a built-in bulb to help transfer fluids and are known as transfer pipets. This type of pipet usually does not have any measurement lines on the tube. Micropipettes are used to deliver very small volumes of liquid. These pipetting devices must be fitted with an appropriate disposable tip. The device is equipped with a piston at the top, which must be depressed before the pipet is filled and when the pipet is drained. It is important to follow the manufacturer’s instructions for use with all pipette and micropipette. Each type of pipet may be slightly different.

CLIA insight: what a medical assistant may perform depends on the test’s CLIA complexity category. MAs generally perform only CLIA-waived tests, so knowing a test’s complexity category - not just the pipetting technique - determines whether the MA can run it.