Quality Assessment/Reference Intervals/Critical Values

Week 11

What is Quality?Quality:In technical usage, quality can have two meanings: 1. the characteristics of a product or service that bear on its ability to

satisfy stated or implied needs2. a product or service free of deficiencies

Quality assessment (quality assurance):all the practices involved in maintaining high quality in our work• monitor all aspects of laboratory testing

– preanalytical factors (like specimen handling) *what else?– analytical factors (like using a buffer of the correct pH when

performing electrophoresis) *what else?– postanalytical factors (like reporting out results) *what else?

The Difference Between Quality Assessment/Assurance (QA) and

Quality Control (QC)

• QA covers all aspects of laboratory work• QC focuses on test results (analytical

phase)– relies heavily on statistics and “Westgard

rules”

Continuous Quality Improvement (CQI)

Also called Total Quality Management (TQM), etc.• used in many different industries, based on

these concepts:

1. customer focus2. management commitment to quality3. training4. process capability and control5. measurement using quality improvement tools

What the ASQ says about CQI/TQM:

• a management approach to longterm success through customer satisfaction

• based on all members of an organization participating in improving processes, products, services and the culture in which they work

(More on this in CLS 4040)

Review: Accuracy vs. Precision• Accurate result: correctly reflects the “true value”

– Imagine a “target” value that should be obtained for an analyte

• Precision occurs when, after repeated analysis, the same result is achieved

Basic StatisticsMean: the average of a set of numbers

For this set: 23, 30, 15 calculate the mean this way: (23+30+15)/3 = 22.67

• A statistical measure of accuracy• Use the mean value to perform other statistical

tests on the group• One indicator of “central tendency”

– Central tendency: distribution of data around a central value

MeanEXAMPLE:• A student was performing

a manual wet chemistry laboratory in which she obtained five replicate absorbance values for her unknown.

• What is the mean absorbance value for the unknown?

Mean

Basic StatisticsMedian: the central number in a group arranged in

sequential orderA statistical measure of accuracy

For this set: 1, 3, 5, 7, 9 the median is 5

Mode: the number that occurs most frequentlyA statistical measure of accuracy

For this set: 97, 98, 99, 100, 97, 95the mode is 97

Basic Statistics—Gaussian Distribution• Mean, median, and mode are all indicators of central

tendency• If mean, median, and mode are all the same number,

the group of numbers has a Gaussian distribution or normal distribution

• gives a bell-shaped curve when the values are plotted against frequency of each value– equal number of results above and below the

highest point of the bell

Basic Statistics—Variance• Variance:

– Symbol: s2

– Indicates how close together, or how precise, the numbers within a group are

• A group of numbers with a large variance has a wide range of values

• A group of numbers with a small variance has numbers that are very close in value

• The smaller the variance of a group of numbers, the more precise they are

VarianceVariance (s2): the average of squares of

deviations from the mean.• tells us about the precision of our set of numbers

Calculate the variance:• find the deviation from the mean for each value,

square each deviation, add the squares together, then divide the total number by (n-1), where n = number of values in the set

Here’s our variance formula:

Variance = s2 = Σ(value – mean)2 n -1

VarianceEXAMPLE:• Calculate the variance of the following

group of glucose values. All values have units of mg/dL:

94, 93, 92, 100, 101, 93, 105, 98, 99, 87

Variance1. Rank the numbers from lowest to

highest. 87 Lowest92

  93  93  94

98  99  100  101

105 Highest 

Basic StatisticsStandard deviation (s): square root of the

variance. This is another way of looking at precision or spread.

Formula for s (or SD)

Standard Deviation

EXAMPLE:• Calculate the standard deviation of the

following group of glucose values. Units are mg/dL:94, 93, 92, 100, 101, 93, 105, 98, 99, 87

Probabilities Associated with Standard Deviation

Statistically, if a sample is analyzed 30 times and the mean and s are established for the results obtained:68.2% of the time, the results will fall within plus or minus (±) 1

s from the mean95.5% of the time, the results will fall within ±2 s from the mean99.7% of the time, the results will fall within ±3 s from the

mean.

Basic StatisticsCoefficient of variation (CV): ratio of standard

deviation/mean

Confidence limit: confidence interval or standard deviation range

• range of numbers centered on the mean• Calculate this way: subtract the SD from the

mean to determine the lower limit of the range, and then add the SD to the mean to determine the upper limit of the range.

Reference Intervals• the usual test result values for a healthy population

Example: the reference range for fasting glucose is usually stated as 70-110 mg/dL

• Samples from a random selection of healthy people will fall within the range of mean +/- 2 s

• reference range for an analyte depends on the particular test method being used, as well as on the patient population

• how does patient population affect reference range?

• reference range for each analyte being reported by a particular clinical laboratory must be determined (or at least validated)

Determining Reference Intervals

1. Make sure the test method/instrument combination is working properly

2. Obtain specimens from healthy people. To determine a reference range “from scratch”, you will need 120 samples from each age/sex group. (For some analytes, age and sex are important, and for others, they are not.)

3. Handle and store the specimens properly

4. Perform the tests– Ideally, the specimens are obtained and tested over a long period of

time (5-10 per day, for instance, not 240 specimens at once).– Spreading out the testing eliminates bias in the results

5. “Crunch” your data. – Determine the mean, standard deviation, CV, etc. – Traditionally, the reference range covers 95% of the test results from

your healthy subjects

Verifying Reference Intervals• reference ranges provided by the

manufacturer of the instruments and reagents can be checked and verified using far fewer test results

• standard approach is to use 20 specimens from healthy individuals (covering both sexes and all ages in the group)

• or use 60 specimens and do statistics

More on Units

What are “units” in the lab?

Examples of Units

mg/dL IU/L mg/dL

mOsm/kg % mmol/L

Number/mL fL mL/min

Critical Values• a test result that is so abnormal that it indicates the

patient is in immediate danger. Critical values are to be phoned to a physician, nurse, or other staff member immediately.

• Also commonly called a “panic value”• The person (Lundberg) who is credited with coming up

with the concept defines critical value this way: “laboratory test result that represents a pathophysiologic state at such variance with normal as to be life-threatening if an action is not taken quickly and for which an effective action is possible”.

• There are critical values for many chemistry, hematology, coagulation, and other tests

HEMATOLOGY CRITICAL VALUES--Examples

 Test Low HighHematocrit 

0-1 week  None  >65%    1 week - 12 years  <30%  >59%    >12 years  <18%  >59% 

Platelet Count  50,000/mm3

 >1,000,000/mm3 

CHEMISTRY CRITICAL VALUES-Examples

Test Low HighBUN  none  >100 mg/dl   

Glucose: (Male)  50 mg/dl  400 mg/dl (Female)  40 mg/dl  400 mg/dl  (Newborn)  40 mg/dl  300 mg/dl 

How Critical Values Lists Are Used

• clinical laboratory scientist running a instrument will see a critical value, rerun that test, and then, if it comes out the same, call the physician or nursing unit

• phone call must be documented (usually beneath the result in the patient’s computer record) with the date, time, and name of the person notified

Vital Values• Some labs call these “significant values”

Example: positive TB results

• Many labs do include some result values that would be characterized as vital values on their critical values lists

Clinical Pathway = Critical Pathway = Care Pathway = Care Map

Definition:Clinical pathways are structured,

multidisciplinary plans of care. Each clinical pathway is designed to guide care for patients with a specific condition over a specific time period, with the purpose of improving outcomes.

Clinical Pathways

• Based on clinical guidelines– Clinical guidelines are consensus statements regarding clinical

decision-making (best practices)• Flowcharts

– Step by step procedures– Decision points leading to different actions

• Outcomes/Reasons for Using Pathways– Eliminate inefficiency– Standardize care– Improve patient health/fewer complications– Cost savings

Components of Clinical Pathways

THE END