Work like a medical laboratory scientist: accept or reject specimens, read chemistry, hematology, microbiology and blood bank results, call critical values, run daily QC and plan your certification.

Clinical Laboratory Practice

## Why it matters

Clinical chemistry is usually the busiest department in a hospital lab. Its analysers run thousands of tests a day on serum, plasma and urine: electrolytes, kidney and liver markers, glucose, lipids, proteins, enzymes, hormones and drugs. An MLS does not diagnose, but must recognize when a set of results **does not make sense** (interference, contamination, a mislabelled tube) and when it **makes urgent sense** (a critical value that needs a phone call now).

> **Scope.** The numbers in this lesson are **illustrative teaching values**, not reference ranges to apply to anyone. Every laboratory establishes and validates its own reference intervals for its methods and population. Interpreting results for a patient is the care team's job; nothing here is medical advice.

## How the analysers measure

- **Ion-selective electrodes (ISE)** measure sodium, potassium and chloride as a voltage across a selective membrane.
- **Photometric (color) reactions** measure many analytes, such as glucose and creatinine, by the absorbance of a reaction product (Beer-Lambert law).
- **Enzymatic rate methods** measure enzyme activity (ALT, AST, ALP) by how fast an absorbance changes.
- **Immunoassays** use antibodies to measure hormones, cardiac markers such as troponin, and drugs.

Each method has known **interferences**: hemolysis, lipemia, icterus, and some drugs. The analyser's HIL indices help the lab decide whether a result can be reported.

## The basic and comprehensive metabolic panels

The **basic metabolic panel (BMP)** is eight tests:

| Test | What it reflects | Illustrative adult value |
|---|---|---|
| Sodium (Na) | Water balance | about 135 to 145 mmol/L |
| Potassium (K) | Heart and muscle function | about 3.5 to 5.0 mmol/L |
| Chloride (Cl) | Acid-base and fluid balance | about 98 to 107 mmol/L |
| Bicarbonate (CO2) | Acid-base status | about 22 to 29 mmol/L |
| Urea nitrogen (BUN) | Protein breakdown, kidney clearance | about 7 to 20 mg/dL |
| Creatinine | Kidney filtration | about 0.6 to 1.2 mg/dL |
| Glucose | Blood sugar | about 70 to 99 mg/dL fasting |
| Calcium | Bone, nerve, muscle | about 8.5 to 10.5 mg/dL |

The **comprehensive metabolic panel (CMP)** adds total protein, albumin, bilirubin, ALP, ALT and AST, which reflect liver function and nutrition. (All values above are illustrative only.)

## Reading patterns, not single numbers

Two useful calculations:

- **Anion gap** = Na - (Cl + HCO3). With Na 140, Cl 104 and HCO3 24, the gap is 140 - 128 = **12 mmol/L**, in the usual teaching range of about 8 to 16. A high gap points the care team toward certain kinds of metabolic acidosis.
- **BUN-to-creatinine ratio.** A ratio well above about 20 is classically associated with reduced blood flow to the kidneys (a "prerenal" pattern), while a ratio near 10 to 15 with both raised fits a problem in the kidney itself.

The lab's interest in patterns is also about **plausibility**:

## Worked example: is this result real?

Three fictional potassium results arrive in an hour:

1. **K 6.8 mmol/L, hemolysis index high, other results normal.** Hemolysis releases potassium from red cells. The MLS reports per policy (usually "hemolyzed, recollect") instead of calling it as a critical value.
2. **K 6.8 mmol/L, no hemolysis, creatinine 4.5 mg/dL, previous K 6.1.** This fits a real, worsening problem consistent with kidney failure. It is a critical value: verify per policy and call it now (lesson `clin-m3-l1-critical`).
3. **K 9.1 mmol/L with calcium 2.0 mg/dL in a patient whose results yesterday were normal.** The pattern (very high K, very low Ca) suggests EDTA contamination. Request a redraw.

The tool behind examples 2 and 3 is the **delta check**: the LIS compares a result with the patient's previous one and flags changes too large to be biologically likely. A delta-check failure prompts the MLS to look for a mislabel, contamination or instrument problem before releasing the result.

## Common mistakes

- Treating a single out-of-range value as a diagnosis instead of a prompt to check the specimen and the pattern.
- Releasing a hemolyzed potassium without a comment.
- Ignoring a delta-check flag because the value is "only slightly high."
- Quoting reference ranges from the internet; the lab's own validated intervals are the only ones that apply to its results.

## Quick recap

- Chemistry analysers use ISE, photometry, enzyme rates and immunoassays, each with known interferences.
- BMP: Na, K, Cl, CO2, BUN, creatinine, glucose, calcium; CMP adds liver tests and proteins.
- Anion gap = Na - (Cl + HCO3); the BUN/creatinine ratio hints at prerenal vs kidney patterns.
- Plausibility checks (HIL indices, delta checks, known contamination patterns) protect the patient.

## Try it

Open the **Do** tab for the quiz: calculate an anion gap and decide whether three results are real.


Understand chemistry analysers and the metabolic panel, calculate an anion gap and tell real results from artefacts.

Clinical chemistry

Basic Metabolic Panel (BMP) / Chem 7 Results Explained

## Why it matters

The **complete blood count (CBC)** is one of the most ordered tests in medicine. Hematology analysers count and size hundreds of thousands of cells in under a minute, but they cannot see everything. A large part of the hematology MLS's skill is knowing when an analyser's numbers need a **blood smear review** under the microscope, and what the smear shows.

> **Scope.** The numbers in this lesson are **illustrative teaching values**, not reference ranges to apply to anyone. Every laboratory establishes and validates its own reference intervals for its methods and population. Interpreting results for a patient is the care team's job; nothing here is medical advice.

## What the analyser measures

Analysers use **electrical impedance** (cells passing through an aperture change its resistance: the pulse size gives cell volume) and **flow cytometry with light scatter and fluorescence** to separate white-cell types. From these they report:

| Parameter | Meaning | Illustrative adult value |
|---|---|---|
| WBC | White cells | about 4.5 to 11.0 x10^9/L |
| RBC | Red cells | about 4.2 to 5.9 x10^12/L |
| Hgb | Hemoglobin | about 12 to 17 g/dL |
| Hct | Packed red-cell fraction | about 36 to 50 % |
| MCV | Mean red-cell volume | about 80 to 100 fL |
| MCH, MCHC | Hemoglobin per cell, and its concentration | MCHC about 32 to 36 g/dL |
| RDW | Variation in red-cell size | about 11.5 to 14.5 % |
| PLT | Platelets | about 150 to 400 x10^9/L |

The **differential** splits white cells into neutrophils, lymphocytes, monocytes, eosinophils and basophils, as percentages and absolute counts. (All values above are illustrative only.)

## The red-cell indices

Two indices are calculated from the measured values, and you can check them by hand:

- **MCV (fL)** = Hct (%) x 10 / RBC (x10^12/L)
- **MCHC (g/dL)** = Hgb (g/dL) x 100 / Hct (%)

A quick plausibility check, the **rule of three**, says that for normal-sized cells, Hgb x 3 is roughly the Hct. When it fails badly, think of interference: a cold agglutinin clumps red cells and gives a falsely low RBC and a falsely high MCV and MCHC; lipemia falsely raises hemoglobin.

## Worked example: reading a CBC

A fictional result: Hgb 9.0 g/dL, Hct 29 %, RBC 4.6 x10^12/L, RDW 17 %, PLT 410.

- MCV = 29 x 10 / 4.6 = **63 fL**: small cells (microcytic).
- MCHC = 9.0 x 100 / 29 = **31 g/dL**: slightly low (hypochromic).
- RDW raised: the cells vary in size.

Small, pale, varied cells fit the classic laboratory picture of iron deficiency, while thalassemia trait often shows small cells with a *normal* RDW and a relatively high RBC count. Telling them apart needs further tests (iron studies, hemoglobin electrophoresis) ordered by the care team. The lab's job is an accurate count, correct flags and a good smear review.

## When to look at a smear

Labs set **smear review criteria**: for example a first-time abnormal result, a very high or low count, or an analyser flag such as "blasts?", "atypical lymphs?", "platelet clumps" or "RBC fragments." On a stained peripheral smear, the MLS looks for:

- **Platelet clumps:** EDTA can make platelets clump in some people, giving a falsely low count (pseudothrombocytopenia). A redraw in a citrate tube often resolves it.
- **Schistocytes:** fragmented red cells, which can signal red cells being sheared in small vessels.
- **Spherocytes, target cells, sickle cells:** shape changes linked to specific conditions.
- **Hypersegmented neutrophils** (six or more lobes), often seen with B12 or folate deficiency and large cells.
- **Blasts:** immature white cells that must be reported and referred for pathologist review.

Any suspicion of blasts or a new serious finding follows the lab's escalation path; it is never left in a comment field alone.

## Common mistakes

- Reporting a low platelet count without checking for clumps.
- Ignoring a failed rule of three.
- Running a CBC on a clotted tube.
- Reading a smear in the thick part of the slide, where cells overlap; the monolayer near the feathered edge is where morphology is reliable.

## Quick recap

- Analysers use impedance and flow cytometry; flags trigger smear review.
- MCV = Hct x 10 / RBC; MCHC = Hgb x 100 / Hct; rule of three: Hgb x 3 is about Hct.
- MCV sorts anemias into microcytic, normocytic and macrocytic patterns for the care team.
- Smears catch platelet clumps, fragments, shape changes and blasts.

## Try it

Open the **Do** tab for the quiz: calculate indices and decide when a smear is needed.


Read a CBC, calculate red-cell indices and know which findings need a blood smear review.

Hematology

Complete Blood Count with Differential Explained

Anemia: Lesson 4 - Clues from the blood smear

## Why it matters

When someone has a serious infection, two questions drive treatment: **what organism is it**, and **which antibiotics will work**. The clinical microbiology lab answers both. Its reports look different from chemistry reports: organism names, colony counts and tables of MICs with S, I and R letters. An MLS has to produce those reports accurately and explain what each part means to the care team.

> **Safety and scope.** Clinical microbiology is done in a biosafety level 2 (or higher) laboratory, under supervision, with training in biosafety and bloodborne pathogens ([CDC BMBL](https://www.cdc.gov/labs/bmbl/index.html), checked on 2026-10-23). Never attempt to culture anything at home. Choosing a treatment is the prescriber's decision; nothing here is medical advice.

## The workflow

1. **Specimen:** blood culture bottles, urine, sputum, wound swab, spinal fluid. Quality matters: a sputum with many squamous epithelial cells is mostly saliva and may be rejected.
2. **Direct examination:** a **Gram stain** sorts bacteria into Gram-positive (purple, thick peptidoglycan wall) and Gram-negative (pink) and describes shape (cocci, rods) and arrangement (clusters, chains). For a positive blood culture, this first result is phoned to the care team quickly.
3. **Culture:** the specimen is plated on media such as blood agar (general growth, hemolysis patterns), chocolate agar (fastidious organisms) and MacConkey agar (Gram-negative rods; lactose fermenters turn pink). Plates incubate, usually overnight.
4. **Identification:** biochemical tests, automated panels, or **MALDI-TOF mass spectrometry**, which identifies an organism from its protein fingerprint in minutes. Molecular panels (PCR) can identify organisms and some resistance genes directly from positive blood cultures.
5. **Antimicrobial susceptibility testing (AST):** measures whether antibiotics inhibit the organism.
6. **Report:** preliminary results are released as they become available, then a final report.

## How susceptibility is measured

- **Broth microdilution** finds the **minimum inhibitory concentration (MIC)**: the lowest antibiotic concentration (in a doubling series such as 0.5, 1, 2, 4, 8 ug/mL) that prevents visible growth.
- **Disk diffusion** places antibiotic disks on a lawn of the organism; the diameter of the clear **zone of inhibition** is measured in millimeters.

The MIC or zone size is then compared with **breakpoints** published by standards bodies (CLSI in the US, EUCAST in Europe) to give an interpretive category: **S** (susceptible), **I** or **SDD** (intermediate, or susceptible-dose-dependent), or **R** (resistant). Breakpoints are specific to the organism, the antibiotic and sometimes the infection site, and they are updated, which is why labs follow the current published edition rather than memorizing them.

## Worked example: reading a report

A fictional urine culture report:

```text
Specimen: Urine, clean catch
Culture: >100,000 CFU/mL Escherichia coli
Susceptibility (MIC, ug/mL):
  Ampicillin            >16   R
  Cefazolin              2    S
  Nitrofurantoin        <=16  S
  Trimethoprim/sulfa    >2/38 R
  Ciprofloxacin          0.25 S
```

How to read it:

- **CFU/mL** is the colony count. Interpretation depends on the specimen and collection method, so the source line matters.
- An MIC number is meaningful only with its interpretation; you **cannot** compare MIC numbers across different antibiotics to pick "the strongest" one, because each drug has its own breakpoint.
- Many labs use **cascade reporting**: broader-spectrum drugs are suppressed when narrower ones test susceptible, to support antimicrobial stewardship. A missing drug may be deliberate.
- Some organisms have **intrinsic resistance** (for example many Gram-negative rods to vancomycin), so those drugs are not tested or are reported as resistant automatically.

## Common mistakes

- Comparing MIC values between different antibiotics as if lower always means better.
- Ignoring the specimen source when reading colony counts.
- Treating a mixed growth of several skin or urogenital organisms as a single pathogen (often it signals contamination and a recollection).
- Using outdated breakpoint tables.

## Quick recap

- Workflow: specimen, Gram stain, culture, identification (often MALDI-TOF), susceptibility, report.
- MIC = lowest concentration that inhibits growth; disk diffusion measures zone diameter.
- Breakpoints from CLSI or EUCAST turn MICs and zones into S, I/SDD or R.
- Cascade reporting and intrinsic resistance explain why some drugs are missing from a report.

## Try it

Open the **Do** tab for the quiz: read a culture and susceptibility report.


Follow a specimen from Gram stain to culture and read a susceptibility report with MICs and breakpoints.

Clinical microbiology

Routine Microscopy – Gram Stain Procedure

ID Laboratory Videos: Antibiotic susceptibility testing

## Why it matters

The blood bank (immunohematology) makes sure that a patient who receives a transfusion gets blood their immune system will not attack. ABO-incompatible transfusion can cause an acute hemolytic reaction that can kill, and most such events trace back to **identification errors**, not test failures. That is why blood bank work is the most procedure-driven area of the lab.

> **Safety and scope.** Blood bank testing is done only in an accredited laboratory, by trained and competency-assessed staff, under the supervision of a medical director. This lesson is knowledge only and is not medical advice.

## ABO and Rh

Red cells carry **antigens** on their surface. In the ABO system:

| Blood group | Antigens on red cells | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |

People make ABO antibodies naturally in early life, which is why ABO mismatch is so dangerous. The **Rh(D)** antigen is either present (Rh-positive) or absent (Rh-negative). Rh-negative people usually make anti-D only after exposure, for example through transfusion or pregnancy.

## Typing: forward and reverse must agree

- **Forward type:** the patient's red cells are mixed with anti-A and anti-B reagents. Agglutination (clumping) shows which antigens are present.
- **Reverse type:** the patient's plasma is mixed with known A1 and B reagent red cells. Agglutination shows which antibodies are present.

The two must **agree**. A group A patient shows agglutination with anti-A (forward) and with B cells (reverse). If they disagree (an **ABO discrepancy**), the lab must resolve it before issuing type-specific blood; until then, the patient receives group O red cells if transfusion cannot wait. Discrepancies have causes such as weak antibodies in newborns and older adults, recent transfusion, or a mislabelled sample. Many hospitals also require a **second, separately drawn sample** to confirm the type before issuing non-O red cells.

## Antibody screen and crossmatch

- **Antibody screen:** the patient's plasma is tested against two or three reagent red cells that carry the clinically important non-ABO antigens (such as Kell, Duffy and Kidd). A positive screen means the lab must identify the antibody and find antigen-negative units, which can take hours.
- **Crossmatch:** a final compatibility check between the patient and a specific donor unit.
  - **Immediate-spin crossmatch:** patient plasma plus donor cells, spun and read at once; it mainly confirms ABO compatibility.
  - **Antiglobulin (AHG) crossmatch:** includes incubation and anti-human globulin to detect IgG antibodies; used when the patient has a clinically significant antibody.
  - **Electronic crossmatch:** a validated computer check of ABO compatibility, allowed only when the patient has a negative screen and two concordant types on record.

A **type and screen** is often ordered before surgery; a crossmatch follows only if blood is likely to be needed.

## Worked example: choosing compatible units

A fictional patient types as **B, Rh-negative** with a negative antibody screen.

- Compatible **red cells**: group B or O (no A antigen, because the patient has anti-A), and Rh-negative.
- Compatible **plasma**: group B or AB (plasma must not contain anti-B, because the patient's cells carry B).

The pattern reverses between red cells and plasma: group O red cells suit everyone (no A or B antigens), while group AB plasma suits everyone (no anti-A or anti-B). In an emergency before the type is known, labs issue O red cells (often O-negative for females of childbearing potential) and AB plasma, per policy.

## Common mistakes

- Issuing type-specific blood while a forward/reverse discrepancy is unresolved.
- Confusing red-cell and plasma compatibility rules.
- Accepting a blood bank sample with any labelling defect. Blood bank labelling rules are the strictest in the lab, and a defect means a new sample.
- Using an electronic crossmatch for a patient with a positive antibody screen.

## Quick recap

- ABO antibodies are naturally present; forward and reverse types must agree.
- Antibody screens detect clinically significant non-ABO antibodies.
- Crossmatch: immediate spin, AHG, or electronic (only when eligible).
- Red cells: O is universal. Plasma: AB is universal. Identification errors are the main danger.

## Try it

Open the **Do** tab for the quiz: match patients to compatible units.


Explain ABO and Rh typing, antibody screens and crossmatches, and choose compatible red cells and plasma.

Immunohematology

Blood Types (ABO and Rh) - Antigens and Antibodies

Pre-Transfusion Testing: Type, Screen, and Crossmatch

Read results from clinical chemistry, hematology, microbiology and the blood bank, and spot results that cannot be real.