What Do the Arrows on My Blood Test Mean?
H, L, asterisks, bold values — a plain-English guide to understanding every flag on your Australian pathology report, and which ones actually matter.
The One Thing to Understand First
Australian pathology labs set their reference intervals — the “normal” range — to include 95% of healthy people. This means that 5% of healthy people — 1 in 20 — will have a result outside the reference range for any given test, purely by statistical chance. If you have a panel of 20 blood tests, it is expectedthat at least one will be mildly flagged, even if everything is completely fine.
This does not mean the flags should be ignored — some are very important. But it does mean that a single mildly out-of-range result, without supporting symptoms or a pattern across multiple tests, is often not clinically significant.
What Every Flag Symbol Means
Different Australian pathology providers use slightly different notation, but these are the most common symbols you will see on a report from services like Sullivan Nicolaides, Sonic Healthcare, Australian Clinical Labs, or a public hospital lab.
High — above the upper reference limit
The result is above the lab's upper limit of the reference interval. May be mildly or significantly elevated. Degree of elevation and which test was flagged determines clinical significance. A very mildly elevated result is often not actionable.
Low — below the lower reference limit
The result is below the lab's lower reference limit. As with H flags, context determines significance. A result that is 2-3% below the lower limit in an otherwise well person is rarely a cause for concern.
Arrows — direction of out-of-range result
Some Australian labs use upward or downward arrows instead of (or alongside) H and L. The same principle applies — the degree of deviation and which marker was flagged determines whether action is needed.
Asterisk — significantly abnormal or critical value
A single asterisk usually indicates a notably abnormal result; a double asterisk or "!!" often indicates a critical value — a result so far from normal that life-threatening pathology must be excluded. Labs phone the GP directly for critical values.
Visual flag — abnormal highlighted for attention
Many Australian pathology reports bold or colour flagged results to draw attention. The degree of boldness or colour intensity may reflect severity, but always check the numerical value rather than relying on formatting alone.
Critical value — requires immediate clinical action
Some reports print "CRITICAL" or "PANIC VALUE" explicitly. The lab will call your doctor immediately. If you see this on an online result and cannot reach your GP, attend an emergency department or urgent care centre.
Why Your Lab's Reference Range May Differ from Another Lab
Every laboratory in Australia establishes its own reference intervals based on the equipment, reagents, and methods it uses — and on the local healthy population it has studied. This is why the “normal” upper limit for ALT at one lab might be 45 U/L and at another lab 55 U/L. Neither is “wrong” — they reflect different methods calibrated to different reference populations.
The Royal College of Pathologists of Australasia (RCPA) provides guidelines and harmonisation frameworks, but individual labs retain flexibility. This means:
Always use the reference range printed on YOUR report — not a number from an online source or a different lab
When comparing results over time, use the same lab — switching labs mid-monitoring can introduce apparent changes that are just methodological differences
Some reference ranges are age- and sex-adjusted — a result flagged in a 20-year-old woman may be within range for a 70-year-old man with the same value
Results That Genuinely Require Prompt GP Review
These are the specific critical value thresholds that Australian pathology labs will typically phone through to your GP. If you see these in an online result before speaking to your doctor, seek same-day advice:
Potassium
Severe cardiac arrhythmia — can cause cardiac arrest
Sodium
Brain swelling (hyponatraemia) or dehydration crisis (hypernatraemia)
Glucose
Diabetic crisis (DKA/HHS) or severe hypoglycaemia
Haemoglobin
Severe anaemia — cardiac compromise in vulnerable patients
Calcium (total)
Hypercalcaemic crisis — confusion, cardiac arrhythmia, kidney failure
INR/PT
Critical bleeding risk, especially in patients on warfarin
Platelets
Spontaneous haemorrhage risk — urgent haematology review
Creatinine
Acute kidney injury — cause must be identified and removed immediately
Troponin
Possible heart attack — emergency department immediately
Lipase / amylase
Acute pancreatitis — may require hospitalisation
Mildly Flagged Results — Common Examples and What They Usually Mean
These are some of the most common mildly flagged results Australian GPs encounter, and what they typically represent in otherwise well patients:
ALT 52 U/L (ref: < 45)
Likely cause: Mildly elevated — commonly seen with fatty liver, recent exercise, alcohol, or simply a normal variantTypical action: Repeat in 3-6 months; avoid alcohol and NSAIDs; no urgent actionMCV 102 fL (ref: 80-100)
Likely cause: Mildly macrocytic — common with alcohol use, B12/folate low-normal, or thyroid dysfunctionTypical action: B12, folate, and TSH testing recommended; no emergencyFerritin 220 µg/L (ref: < 200)
Likely cause: Borderline high — often inflammation, metabolic syndrome, or fatty liver rather than iron overloadTypical action: Check CRP, iron saturation, LFTs; repeat in 3 monthseGFR 58 mL/min (ref: ≥ 60)
Likely cause: Just below lower limit — can be dehydration, age-related, or early CKDTypical action: Repeat in 3 months + urine ACR; avoid NSAIDs; no acute change neededNeutrophils 1.9 × 10⁹/L (ref: 2.0-7.5)
Likely cause: Mildly low — can be normal variant, especially in people of African ancestry (benign ethnic neutropenia)Typical action: Repeat FBC; if isolated and stable, often no action requiredCholesterol 5.3 mmol/L (ref: < 5.2)
Likely cause: Borderline — marginal excess; the LDL/HDL breakdown and cardiovascular risk score matter more than total cholesterol aloneTypical action: Full lipid panel + 5-year CVD risk calculation by GP; dietary reviewTrends Matter More Than Single Results
One of the most important principles in blood test interpretation is the importance of trends over time rather than single-point snapshots. Consider these scenarios:
eGFR: 72 → 68 → 64 → 59 over 3 years
Consistent, progressive decline — even though each individual result is within or near normal, the trend suggests accelerating CKD. Your GP should investigate and intervene.
eGFR: 58 → 72 → 66 → 70 (stable over 2 years)
The initial 58 was likely transient (dehydration, acute illness). Stable at ~68-72 = no progressive disease. Reassuring.
ALT: 28 → 31 → 44 → 67 over 18 months
Rising ALT trend with a current abnormal value warrants investigation even though early results were fine. The trend tells the story.
Ferritin: 18 → 22 → 25 → 28 (all near-low but stable)
Low-normal ferritin that is stable over time, in an asymptomatic person, may not require aggressive treatment — just monitoring and dietary review.
Your Action Checklist When You Get Flagged Results
Don't panic about a single mild flag
A single mildly flagged result in an otherwise healthy person with no symptoms is often a statistical artefact or transient variation. Check: Is the deviation large or small? Is this the first time? Are there symptoms?
Read the specific result — not just the flag
Look at the actual number and how far outside the range it is. A creatinine of 112 µmol/L flagged H (upper limit 110 in that lab) is very different from a creatinine of 300 µmol/L flagged H. The same H flag can mean very different things.
Check if the lab has contacted your doctor
For critical values, the lab will phone your GP's rooms directly. If you received results online before speaking to your doctor, your GP may already be aware. Call the surgery to check if there has been a message from the lab.
Book a GP appointment for unexplained flags
Any flagged result you don't understand, or that does not match a known condition, should be discussed with your GP. Bring the printed or downloaded result. Your GP can usually tell you on the spot whether it needs follow-up or not.
Consider fasting status, timing, and recent events
Blood glucose rises after eating. Creatinine rises after intense exercise or eating lots of meat. Ferritin rises with any illness. ALT rises after alcohol or certain medications. Many flags have a simple, innocent explanation that disappears on repeat testing.
Don't treat yourself based on blood test results
A low ferritin does not automatically mean you need iron supplements — the cause and clinical context matter. A low vitamin D does not automatically require high-dose supplementation. Always get professional guidance before starting supplements based on results alone.
Reading Your Australian Pathology Report — Format Guide
Most Australian pathology reports share a similar layout. Here is how to navigate them:
Patient details (top left)
Name, DOB, Medicare number, date collected, date reported, lab ID number. Check these are correct before reading any result.
Requesting doctor
Who ordered the test. If you are seeing multiple doctors, results go to the requester — not automatically to all treating doctors.
Test name and result
Left column: test name. Middle column: result. Right column: reference interval (your lab's normal range). The flag (H, L, *) appears next to the result.
Units
Results are reported in specific units (mmol/L, µg/L, U/L, g/L, etc.). Units are critical — a glucose of 5.5 mmol/L is normal; 5.5 mg/dL would be critically low (a different unit system rarely used in Australia).
Free-text comments
Many labs add interpretive comments below results — especially for complex results like blood films, protein electrophoresis, or unusual patterns. These are written by the pathologist and are clinically valuable — read them.
Pathologist signature and contact
The reporting pathologist's name and contact appear at the bottom. Your GP can call them directly for expert interpretation of unusual results.
Related Reading
Get Your Blood Test Explained in Plain English
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This page provides general educational information about how to interpret flags and symbols on Australian pathology reports. It is not a substitute for professional medical advice. Always discuss your blood test results with your GP, who can interpret them in the context of your full medical history. SmarterBlood does not provide medical care or diagnoses.