Interpreting Common Lab Tests for OPRA: FBC, U&E, LFTs and CRP

Full blood count: let MCV do the sorting

A full blood count (FBC) question is almost always an anaemia question, and the fastest way into it is mean cell volume (MCV) — it sorts anaemia into three mechanistically different groups before you've looked at anything else.

Microcytic anaemia (low MCV) usually means iron deficiency, but can also reflect thalassaemia or, less commonly, anaemia of chronic disease. Macrocytic anaemia (high MCV) points to vitamin B12 or folate deficiency, or to causes unrelated to deficiency such as alcohol excess or hypothyroidism. Normocytic anaemia (normal MCV) is the least specific of the three and includes acute blood loss, anaemia of chronic disease, and early or mixed deficiency.

MCV narrows the question; it doesn't answer it. The second-line tests are what confirm the mechanism.

• Ferritin — low ferritin is specific for iron deficiency; a normal or high ferritin doesn't rule it out, because ferritin is also an acute-phase reactant that rises with inflammation and can mask a coexisting deficiency.
• Total iron-binding capacity (TIBC) / transferrin saturation — raised TIBC with a low transferrin saturation supports true iron deficiency; a low or normal TIBC points more towards anaemia of chronic disease.
• B12 and folate — low levels confirm a macrocytic deficiency anaemia; a raised MCV with normal levels should prompt a look at alcohol intake, thyroid function, or the medicine list (methotrexate, some antiretrovirals, hydroxyurea).
• Reticulocyte count — a raised reticulocyte count shows the bone marrow is responding appropriately (as after acute blood loss or haemolysis); a low or inappropriately normal count in the presence of anaemia points to underproduction.

Electrolytes: which results actually need a phone call

Sodium and potassium are reported constantly, and most abnormal results are mild and don't require same-day action. The OPRA-relevant skill is recognising the minority that do — because the exam is testing judgement, not just recall of a reference range.

• Potassium above about 6.0–6.5 mmol/L, or any potassium with new ECG changes (peaked T waves, widened QRS, absent P waves), is a medical emergency — cardiac arrhythmia and arrest are the risk. This is the electrolyte result most likely to appear as an urgent-action stem.
• Potassium below about 2.5 mmol/L carries a similar arrhythmia risk in the opposite direction, particularly in a patient on digoxin, where hypokalaemia increases digoxin toxicity risk at a given digoxin level.
• Sodium moved rapidly, in either direction, is more dangerous than the absolute number — a sodium of 125 mmol/L that developed over two days is a different clinical problem to the same number present for months, because rapid correction of chronic hyponatraemia itself risks osmotic demyelination.
• A modestly low or high potassium in a patient on a relevant medicine (an ACE inhibitor/ARB/spironolactone for hyperkalaemia; a loop or thiazide diuretic for hypokalaemia) is the everyday version of this question — recognising the causative drug class is usually the actual point being tested.

Liver function tests: hepatocellular vs cholestatic

A liver function test (LFT) panel usually reports ALT and AST (transaminases), ALP (alkaline phosphatase), GGT and bilirubin together, and the pattern across them — not any single number — tells you what kind of liver injury is occurring.

A hepatocellular pattern shows ALT and AST raised disproportionately to ALP, reflecting damage to liver cells themselves. Drug-induced hepatotoxicity (paracetamol overdose, isoniazid, methotrexate), viral hepatitis and alcoholic hepatitis typically produce this pattern.

A cholestatic pattern shows ALP (and usually GGT) raised disproportionately to the transaminases, reflecting obstruction to bile flow. Gallstones, some medicines (including certain antibiotics and antipsychotics), and infiltrative liver disease typically produce this pattern.

Bilirubin can rise in either pattern once liver function is sufficiently impaired, which is why it's the least specific of the panel on its own — jaundice tells you something is wrong, but the transaminase-to-ALP ratio is what tells you which kind of problem it is.

• GGT rising alongside ALP supports a genuinely hepatic or biliary cause for a raised ALP, since ALP alone can also come from bone.
• A very high ALT (many multiples of the upper limit of normal) in an acute presentation should raise paracetamol overdose specifically, given how common and how treatable it is.
• Albumin and INR/prothrombin time reflect the liver's synthetic function rather than current injury, and are more useful for judging severity and chronicity than for classifying the pattern.

Inflammatory markers: CRP and ESR aren't interchangeable

CRP and ESR are both non-specific markers of inflammation, elevated by infection, autoimmune disease and malignancy alike — neither tells you the cause. What OPRA tends to test is the difference in how they behave over time.

CRP is produced directly by the liver in response to inflammatory cytokines, rises within hours of an inflammatory trigger, and falls quickly once the trigger resolves — which makes it useful for tracking whether treatment (an antibiotic course, for example) is working. ESR reflects how quickly red cells settle in a sample, is influenced by plasma proteins including fibrinogen, rises more slowly over days, and falls more slowly too — which makes it less useful for tracking a fast-moving acute illness but still used in some chronic inflammatory conditions.

A CRP that is already falling on day two or three of antibiotic therapy is a reasonable sign of response; an ESR is the wrong test to expect to have moved meaningfully over the same interval.

INR: two different reasons OPRA asks about it

International normalised ratio (INR) standardises prothrombin time results across laboratories and is the number used to monitor warfarin therapy. OPRA tests it in two distinct contexts, and telling them apart is the actual skill.

The first is routine warfarin monitoring: is this INR within the patient's target range (commonly 2–3 for most indications), and if not, is a dose change or a hold needed, and is there an interacting medicine or dietary change that explains the drift? The second is bleeding risk in a patient not on warfarin, or an unexpectedly deranged INR that instead reflects liver synthetic dysfunction (the liver makes clotting factors) or vitamin K deficiency — a raised INR here is a marker of illness severity, not a dosing parameter to titrate.

A stem describing a patient on warfarin with a specific target range is asking a monitoring question. A stem describing an unwell patient with no mention of warfarin and a raised INR is very likely asking about hepatic synthetic function instead.

Practise this topic

Lab interpretation rewards pattern recognition built up over many questions, not memorising a single table. Practise more Biomedical Sciences OPRA questions with ClinicalStem's OPRA question bank, or work through the Crash Course modules on inflammatory markers, cardiac biomarkers, liver serology and thyroid function for focused practice on each.

Reading a result in the context of the drug list

A pharmacist rarely sees a lab result without also seeing the medicine list, and OPRA scenarios are built the same way — the point of the question is usually connecting the two.

A falling potassium in a patient on a loop diuretic, a rising creatinine after starting an ACE inhibitor, a low platelet count on long-term sodium valproate, a raised INR in a patient recently started on an antibiotic that inhibits warfarin metabolism — in each case the result on its own is unremarkable, and the medicine list is what makes it make sense. Reading the drug history alongside the pathology is the applied skill OPRA is testing, more than the reference range itself.

Expected change vs a result that needs action

Some drug-related lab changes are expected and don't need intervention; others need it. Confusing the two in either direction is a common error.

A creatinine rise of up to about 30% after starting an ACE inhibitor or ARB reflects the expected reduction in intraglomerular pressure and is not itself a reason to stop — a rise beyond that, or new hyperkalaemia, is what prompts dose reduction or specialist review. A statin causing a mild, asymptomatic transaminase rise is common and doesn't automatically mean stopping the statin; a marked rise, or a rise with symptoms, does. Recognising which category a described change falls into — expected and monitored, or a genuine trigger for action — is frequently the entire question.

When a normal result doesn't mean what it looks like

A single normal-range result can still be clinically wrong for that particular patient, and OPRA uses this deliberately.

• A 'normal' ferritin in a patient with clear signs of iron deficiency and an active inflammatory process may reflect ferritin's behaviour as an acute-phase reactant rather than genuinely adequate iron stores.
• A potassium sitting at the low end of the reference range in a patient on digoxin is still clinically relevant, because hypokalaemia increases digoxin toxicity risk even without the potassium itself being frankly abnormal.
• An INR within target range doesn't exclude a clinically important interaction that has only just been introduced — the next check is what will show it, not the current one.

Do I need to memorise every reference range for OPRA?

Knowing the commonly tested ranges (sodium, potassium, creatinine, INR) is useful, but OPRA more often tests whether you can interpret a result in context — is this expected, does it need action, what does it mean alongside the medicine list — rather than asking you to recall a number in isolation.

What's the fastest way to approach an anaemia question?

Start with MCV. It sorts anaemia into microcytic, normocytic and macrocytic, which tells you which second-line test — ferritin/TIBC for microcytic, B12/folate for macrocytic, reticulocytes for normocytic — is actually relevant to reach for next.

How do I tell a hepatocellular LFT pattern from a cholestatic one?

Compare the transaminases (ALT, AST) to ALP. Transaminases raised disproportionately to ALP is a hepatocellular pattern, reflecting liver cell damage. ALP (and usually GGT) raised disproportionately to the transaminases is a cholestatic pattern, reflecting obstructed bile flow.

Why does OPRA distinguish CRP from ESR if they're both inflammatory markers?

Because they behave differently over time. CRP rises and falls within days, making it useful for tracking short-term treatment response; ESR moves more slowly, so it isn't the right marker to expect to have changed over a short interval even when treatment is working.

Is a raised INR always a warfarin problem?

No. In a patient on warfarin, a raised INR is usually a monitoring and dosing question. In a patient not on warfarin, an unexpectedly raised INR more often points to impaired liver synthetic function or vitamin K deficiency, since the liver produces most clotting factors.