Clinical chemistry lab

Light - based analytical methods: Absorbance spectrophotometry

light absorbed by the reaction product; absorbance increases with concentration when beer’s law applies

Light - based analytical methods: Fluorometry

emitted light after excitation

Light - based analytical methods: Nephelometry

Light scattered by particles, typically measured at an angle

Light - based analytical methods: Reflectance

light reflected from a reaction surface or dry chemistry layer

Light - based analytical methods: Chemiluminescence

Light produced by a chemical reaction; best sensitivity compared to other methods discussed

Light - based analytical methods: Monochromator

isolates the desired wavelength from polychromatic or white light

Light - based analytical methods: Beer’s law

depends on molar absorptivity and path length remaining constant

Light - based analytical methods: Bichromatic analysis

can help correct for background or instrument - related absorbance interference

Light - based analytical methods: A method

May fail even when the optics are fine if the reagent, blank, temperature, or mixing is abnormal

Light - based analytical methods: White optical diffuser

reflects light in reflectance spectrophotometry

ISE / Potentiometry

Respond preferentially to selected anions or cations

Na⁺, K⁺, Cl⁻

Electrode deterioration, membrane contamination, bubbles, bad calibration, wrong specimen additive

Amperometry

measures electrical current produced by an oxidation or reduction reaction

PO₂

Membrane damage/interference, diffusion problem, electrode problem, incorrect applied potential

Dye-binding absorbance

Negatively charged dye (coumassie blue) is dissolved in acid and binds to positively charged proteins

Albumin with bromocresol green

Dye degradation/contamination, incorrect pH, reagent blank problem, poor mixing

Colorimetric reaction

Creatinine by Jaffe reaction

Reagent deterioration, timing/pH errors, interfering chromogens, poor blank correction

Coupled enzymatic absorbance

BUN; total CO₂ uses coupled enzymatic chemistry

Cofactor/enzyme degradation, abnormal reagent blank, temperature or mixing problems

Scatter / emitted-light methods

Nephelometry, fluorometry, chemiluminescence

Dirty optics, wrong wavelength/filter, reagent instability, signal quenching or background

Purpose of analytical methods

The point is to recognize the analytical principle and the kinds of failures that can affect any assay using that principle.

Analyzer configurations: Random access

Different tests can be run on different specimens as needed

Analyzer configurations: Continuous flow

Specimens travel through a common flowing stream

Analyzer configurations: Centrifugal

specimen / reagent mixtures are held in rotor chambers

Analyzer configurations: Discrete

Each reaction has its own vessel or reaction space

Analyzer configurations: The ACE Axel

is a discrete, random-access analyzer

Specimen handling: Barcodes

identify specimens and connect them to test orders

Specimen handling: Primary tubes & Secondary cups

Primary tubes reduce manual transfer; secondary cups may be needed for low volume

Specimen handling: Affects

Tube size, gel separator position, dead volume, and aspiration can all affect testing

Specimen handling: loading

Continuous loading supports high-throughput workflows

Sample / fluid-path problems 1

Carry over can transfer analyte or reagent into the next reaction

Ex.) a very high concentration from one sample causing a falsely high value in subsequent sample

Sample / fluid-path problems 2

Probe contamination, inadequate washing, or incomplete rinsing can bias results

Sample / fluid-path problems 3

Air bubbles can interfere with aspiration or electrode contact

Sample / fluid-path problems 4

Insufficient sample volume can cause aspiration errors

Poor mixing can produce unusual QC or patient results

Reaction - environment problems: 1

Incorrect temperature changes reaction rates and enzyme activity

Reaction - environment problems: 2

Reusable cuvettes require adequate cleaning and maintenance

(remember that contamination can cause bias results)

Reaction - environment problems: 3

Reagent storage and onboard stability matter

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Incorrect reagent lot information, expired material, or

calibration mismatch can cause systematic shifts

Reaction - environment problems: 4

Humidity and ambient conditions may affect analyzer performance within vendor limits

Reagent problems: Degraded enzyme or cofactor

Might see: Weak reaction, low rate, abnormal blank, poor recovery

Examples: NADH/NADPH-based coupled assays; enzymatic total CO₂; other enzyme-rate methods

Reagent problems: Contaminated or deteriorated color reagent

High/low blank, unexpected absorbance, bias in patient results

Dye-binding or colorimetric methods such as albumin or creatinine chemistry

Reagent problems: Incorrect pH / buffer condition

Altered color development or enzyme activity

Dye-binding, Jaffe, and enzyme-based reactions

Reagent problems: Improper storage / expired reagent

Drift, low activity, instability, repeated QC failure

Can affect essentially any chemistry reagent

Reagent problems: New lot not aligned with calibration

QC shift or systematic patient bias

Recalibration / lot verification may be required

Reagent problems: Inadequate mixing or delivery

Erratic precision, incomplete reaction, unusual QC

Pump/syringe/probe delivery or mixing problem

Reagent problems: Carryover / wash failure

Next sample biased after a high sample or concentrated reagent

Look for sequence-dependent error, not just one isolated value

Potentiometry (Electrochemistry)

• Measures voltage (potential difference) between electrodes with essentially no current flow

• Used for pH and ion-selective electrodes

• Reference electrode supplies a stable potential

• Indicator/ISE electrode changes potential in response to the target ion

• Calibration converts electrode response into a reportable concentration

Amperometry (Electrochemistry)

• A fixed voltage is applied to drive oxidation or reduction

• The resulting current is measured.

• Current is proportional to analyte concentration

• Classic clinical application: PO₂ measurement

• Working electrode = site of redox reaction; reference electrode stabilizes potential

Electrodes to recognize: 1

Na⁺ (sodium): glass membrane

Electrodes to recognize: 2

K⁺ (potassium): valinomycin neutral ion carrier (ionophore)

Electrodes to recognize: 3

Cl⁻ (chloride): liquid ion-exchange / electroactive salt membrane

Electrodes to recognize: 4

pH: glass membrane responsive to H⁺ activity.

Electrodes to recognize: 5

PO₂ (partial pressure of oxygen): amperometric oxygen sensor with a gas- permeable membrane

Selectivity concepts

• SE membranes respond preferentially—not perfectly— to a target ion

• Interfering ions are described by a selectivity coefficient

• Lower selectivity coefficient = better selectivity for the intended ion

• The liquid junction / salt bridge completes the electrical circuit while separating the sample from the reference electrode

• PCO₂ measurement depends on CO₂ diffusion causing a change in H⁺ within an internal electrolyte layer

pO₂ electrode = amperometry

• Also called a Clark-type oxygen electrode

• Ag/AgCl anode + platinum wire cathode

• Fixed negative potential is applied; course wording uses −0.65 V

• O₂ diffuses through a selectively permeable membrane to the platinum cathode.

• O₂ is reduced at the cathode; the resulting current is proportional to pO₂

• Recognition clue: current + platinum cathode + membrane → pO₂, not Na⁺, pCO₂, or pH

pCO₂ electrode: gas-sensing

• CO₂ diffuses through a membrane into a thin internal electrolyte layer

• Internal electrolyte contains bicarbonate and chloride salts

• CO₂ forms carbonic acid, then dissociates and increases H⁺ concentration

• The internal pH electrode detects the pH/H⁺ change

• Key relationship: higher pCO₂ → higher H⁺ in the internal layer

• Recognition clue: pCO₂ is tied to H⁺ concentration, not current, resistance, or chloride ion concentration

Blood gas electrodes: pO₂ and pCO₂

pO₂ = amperometric current; pCO₂ = CO₂-driven pH/H⁺ change

pH / H⁺ measurement

• pH is measured potentiometrically as hydrogen ion activity

• A pH indicator electrode includes a H⁺-sensitive glass membrane with an internal Ag/AgCl element

• Course/exam wording may identify Ag/AgCl in the pH electrode system

• Reference electrode gives the stable comparison point; calomel/Hg-HgCl₂ is a classic reference electrode option

• Do not confuse the stable reference electrode with the H⁺-responsive indicator system

Cl⁻ electrode membrane

• Chloride uses a liquid ion-exchange membrane

• Selective membrane contains arsonium salt / electroactive salt in the course wording

• It responds to chloride activity through the membrane phase

• A Cl⁻ electrode problem can distort electrolyte patterns

• If Cl⁻ is overestimated, the calculated anion gap becomes falsely low

Na⁺ / Cl⁻ correlation

• Na⁺ is the major extracellular cation; Cl⁻ is the major extracellular anion

• They often move together to preserve electroneutrality and osmotic balance

• Unexpected Na⁺/Cl⁻ mismatch can be a clue to specimen, calibration, or electrode problems

• Do not expect K⁺ or HCO₃⁻ to track with Na⁺ in the same simple way

What happens during measurement

Sample is delivered to the measuring chamber

Na⁺, K⁺, and Cl⁻ electrodes contact the specimen

Each selective membrane develops an electrical potential related to ion activity

The analyzer compares each signal with the reference electrode

Stored calibration converts the electrical signal into a concentration

Calibration logic

Known calibrators establish expected electrode response and slope

A bad slope can reflect electrode deterioration, contamination, calibrator problems, or fluid-path issues.

Calibration does not replace QC.

Calibration logic 2

A new reagent lot or new test method requires calibration

Persistent QC shift or trend (e.g., 10x violation) should trigger troubleshooting and calibration, not automatic reporting

WESTGARD rules

1₂s = warning; investigate the pattern before rejecting solely on this rule

1₃s = reject.

2₂s = reject.

4₁s = reject.

10x = reject; points to systematic error.

Pattern recognition: Shift

Results suddenly move to a new side / level of the mean

Pattern recognition: Trend

Results progressively rise or fall

Pattern recognition: Random error

Unpredictable scatter

Pattern recognition: Systematic error

Consistent directional bias

Na⁺

Reference range: 135–145 mmol/L

Critical values: <120 or >160

Method of measurement: ISE

K⁺

Reference range: 3.5–5.0 mmol/L

Critical values: <3.0 or >7.5

Method of measurement: ISE

Cl⁻

Reference range: 98–107 mmol/L

Critical values: <90 or >110

Method of measurement: ISE

Total CO₂ / HCO₃⁻

Reference range: 23–30 mmol/L

Critical values: <10 or >40

Method of measurement: Enzymatic

Specimen issues with K⁺

Delayed separation from cells can falsely increase

potassium.

Hemolysis can markedly increase potassium and may

make the result unacceptable

Specimen issues with Na⁺

A sodium-containing anticoagulant can falsely increase

sodium.

Specimen issues: Total CO₂ / HCO₃⁻

Total CO₂ is especially sensitive to handling because

CO₂ can be lost before analysis

Specimen issues: general

Wrong tube, insufficient volume, contamination, or

aspiration problems can create misleading results.

Questions before reporting

• Is QC acceptable?

• Was the correct specimen/tube used?

• Was the specimen separated and stored appropriately?

• Is hemolysis or another visible interference present?

• Does the result agree with related analytes and prior

• results?

• Is this a one-patient issue or a batch-wide pattern?

Anion gap

Formula: Na⁺ − (Cl⁻ + HCO₃⁻).

Reference range: : 8–16 mmol/L

High gap = unmeasured anions / added acids

Low gap = uncommon and may be clinical or analytical

Batch logic

A single low gap can be a real patient finding.

Many low gaps in unrelated patients suggest a

system problem

Potential analytical causes include Na⁺

underestimation or Cl⁻ / HCO₃⁻ overestimation

Look at QC, calibration, electrode condition, recent

maintenance, and lot changes

Result acceptability: a repeatable decision process

1. Check QC: if the run is out of control, patient results are not reportable.

2. Check specimen integrity: collection tube, hemolysis, delayed separation, volume, and handling.

3. Check analytical limits: reportable range, flags, and critical values.

4. Evaluate internal consistency: do Na⁺, Cl⁻, HCO₃⁻, K⁺, BUN, and creatinine make sense together?

5. Compare with prior results when available.

6. Decide whether to report, repeat, troubleshoot the analyzer, or recollect the specimen.

Azotemia patterns: Pre Renal

Problem before the kidney: decreased renal perfusion with initially intact renal tissue.

BUN often rises more than creatinine because urea reabsorption increases when flow is reduced.

BUN:creatinine ratio is often >20:1.

Common settings: dehydration/volume loss, hemorrhage, reduced cardiac output.

May improve rapidly if perfusion is restored early.

Azotemia patterns: Intrinsic Renal

Problem within the kidney: glomerular, tubular, interstitial, or vascular injury.

BUN and creatinine both rise; creatinine tends to rise more proportionally.

BUN:creatinine ratio is often about 10–15:1.

Tubular injury reduces normal urea reabsorption.

Electrolyte or acid–base abnormalities may accompany renal dysfunction.

Azotemia patterns: Post Renal

Problem after the kidney: obstruction to urine flow.

BUN and creatinine can both increase; ratio is variable

Early obstruction may resemble pre- renal; prolonged obstruction can

resemble intrinsic renal injury.

Consider urinary obstruction and reduced urine output

Clinical context and imaging are often needed.

Potentiometry & amperometry

Potentiometry = voltage

Amperometry = current

amperometric Clark electrode

pO₂: amperometric Clark electrode; Ag/AgCl anode + platinum cathode + membrane; current reflects O₂ reduction

fundamental pH measurement system

pH/H⁺ systems use a H⁺-sensitive glass membrane with Ag/AgCl internal element; classic references include calomel/Hg-HgCl₂.

clinical chemistry and laboratory troubleshooting

Cl⁻ electrode membrane contains arsonium/electroactive salt; Cl⁻ overestimation lowers anion gap.

Na⁺ and Cl⁻ should generally correlate; repeated mismatch suggests specimen/analyzer review