Clinical Chemistry: Glucose Metabolism, Testing Protocols, and Specimen Integrity

Comprehensive flashcards covering glucose physiology, testing protocols (GTT, 2-hr PC, fasting limits), HbA1c requirements, ketone testing, tube types, glycolysis rates, quality control rules, and CSF glucose interpretations.

What is the oral glucose challenge dose administered to pregnant patients during gestational diabetes screening?

$$50\,\text{g}$$

What is the established reference range for fasting blood glucose?

Approximately $4 - 6\,\text{mmol/L}$

What is the general reference range for random blood glucose, and what upper limit should it typically not exceed?

Approximately $4 - 8\,\text{mmol/L}$ (or $6 - 8\,\text{mmol/L}$), typically not exceeding $11\,\text{mmol/L}$.

What is the standard fasting duration required for a fasting glucose specimen?

Approximately $8\,\text{hours}$

What is the maximum fasting time allowable for glucose collection before hepatic glycogenolysis begins altering baseline results?

$$16\,\text{hours}$$

How long must a patient fast when undergoing testing for a phospholipid or lipid panel?

$$12\,\text{to}\,14\,\text{hours}$$

What time frame of long-term glycemic control does a hemoglobin A1c test reflect?

Approximately $8\,\text{to}\,12\,\text{weeks}$ (or $2\,\text{to}\,3\,\text{months}$ / about $60\,\text{days}$)

What sample type and collection tube are required for performing a hemoglobin A1c assay?

EDTA collection tube containing whole blood (at least half full).

According to the Vitros analyzer protocol, how long must an EDTA whole blood tube be mixed prior to running hemoglobin A1c?

$$15\,\text{minutes}$$

What is the stated normal reference range for hemoglobin A1c?

Approximately $4 - 6\%$

What clinical conditions serve as patient-related sources of error for hemoglobin A1c measurements?

Any factor altering red blood cell lifespan, including anemias, high red cell turnover, hemoglobinopathies/variants, blood transfusions, and snakebite-induced hemolysis.

Which of the three ketone bodies is the most predominant form in circulation?

Beta-hydroxybutyrate

What percentage of total circulating ketone bodies does acetone account for?

$$2\%$$

What is the major pre-analytical source of error when collecting and handling specimens for ketone measurement?

Evaporation due to volatility when tubes are left uncapped.

What are the critical low and critical high cutoff values for blood glucose?

Critical low is $< 2.0\,\text{mmol/L}$; critical high is $> 28.0\,\text{mmol/L}$.

Why must a Levey-Jennings quality control chart display one more decimal place than the reported patient analyte value?

To avoid rounding errors that could erroneously alter whether a QC result falls inside or outside of $2\,\text{SD}$ or $3\,\text{SD}$ limits.

What is the acceptable time threshold to separate serum from cells in an unseparated red-top tube for glucose analysis?

Ideally within $30\,\text{minutes}$, with an upper institutional limit of $60\,\text{minutes}$.

How does the rate of glycolysis compare between red-top, SST, PST, and sodium fluoride tubes during the first hour after collection?

The rate of glycolysis is identical across all tubes during the first hour.

How does the additive in a gray-top tube affect glycolysis from collection up to 72 hours?

Normal glycolysis occurs for the first hour, slows down between $1\,\text{and}\,4\,\text{hours}$, fully stops by $4\,\text{hours}$, and can resume after approximately $72\,\text{hours}$ when the additive is exhausted.

Under what specific clinical circumstance is a gray-top tube chosen over an SST or PST for glucose measurement?

When blood must be transported uncentrifuged over long distances and immediate centrifugation/separation cannot occur.

What is the expected hourly rate of glycolysis in an uncentrifuged whole blood specimen at room temperature?

Approximately $5 - 7\%$ per hour

If an unspun specimen with an initial glucose level of $10\,\text{mmol/L}$ sits for 24 hours at room temperature, what is the expected resulting glucose concentration?

$0\,\text{mmol/L}$ (all glucose is metabolized by the cells)

What is the normal physiological concentration of cerebrospinal fluid (CSF) glucose relative to serum glucose?

CSF glucose is approximately $60\%$ of the concurrent serum glucose concentration.

What condition is suspected when a patient's CSF glucose level drops significantly below 60% of their concurrent serum glucose?

Bacterial meningitis

What is the laboratory processing priority for a CSF specimen relative to routine testing?

CSF is an immediate top priority ('stop, drop, and process') that is superseded only by a massive transfusion bleed protocol.