A Software Engineer at Lumiradx plays a critical role in developing and scaling next-generation point-of-care diagnostic platforms. Because the company’s products directly impact patient health and clinical decision-making, the software you build must meet the highest standards of safety, reliability, and precision. You will work at the intersection of biology, hardware, and cloud technology, ensuring that diagnostic instruments process patient samples accurately and deliver rapid results. In this role, your code will interface directly with embedded systems, optical sensors, and microfluidics, or manage the highly secure cloud infrastructure that transmits diagnostic data to healthcare providers. This requires a deep understanding of low-level programming, robust system architecture, and rigorous testing methodologies. The engineering team at Lumiradx solves complex, multi-disciplinary challenges where software scalability and hardware constraints must be carefully balanced. Joining Lumiradx means contributing to a mission-driven environment where your technical output has a tangible impact on global healthcare delivery. Whether you are optimizing firmware for a handheld diagnostic device or designing secure databases for clinical data, your work will help make diagnostic testing more accessible, affordable, and efficient worldwide.
Recruiter Screen
reportedInitial screening by a recruiter to assess baseline technical competencies.
What to demonstrate
- Initial screening by a recruiter to assess baseline technical competencies
- Depth in Software Design Principles
How to prepare
- Be able to walk your CV end to end in two minutes, and say why this company specifically.
- Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.
Written Technical Assessment
reportedA written test covering programming principles, database design, or specialized topics.
What to demonstrate
- A written test covering programming principles, database design, or specialized topics
- Depth in Software Design Principles
How to prepare
- Answer aloud and timed: Solve a series of bitwise operations and explain how you would isolate specific bits in a register.
- Answer aloud and timed: Explain the difference between stack and heap memory allocation, and how pointer errors can lead to memory leaks in embedded systems.
Technical Interviews
reportedVideo or face-to-face interviews involving live coding exercises and system design reviews.
What to demonstrate
- Video or face-to-face interviews involving live coding exercises and system design reviews
- Depth in Software Design Principles
How to prepare
- Answer aloud and timed: Given a pointer to a struct, how do you safely modify its members without causing undefined behavior?
- Answer aloud and timed: Design a database schema to track patient diagnostic results, device status, and operator logs.
Behavioral Evaluations
reportedDiscussions about past projects, contributions, and design choices.
What to demonstrate
- Discussions about past projects, contributions, and design choices
- Depth in Software Design Principles
How to prepare
- Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
- Re-read the description of the behavioral evaluations above and write down what you would ask to confirm before it.
PracHub editorial advice for the preparation topics above.
Brush up on C fundamentals
Even if your primary language is C# or Java, if you are interviewing for a device-related team, expect questions on C pointers, memory management, and bitwise operations. Do not assume you will have an IDE to help you; practice writing clean code on paper or a whiteboard.
Understand the product line
Take the time to research Lumiradx's point-of-care diagnostic instruments. Showing an understanding of how software interfaces with biochemistry and optical sensors to deliver rapid test results will demonstrate genuine interest and business alignment.
Prepare for one-way video stages
Some candidates report encountering one-way video screens or assessments during the early stages of the process. Practice speaking clearly, maintaining eye contact with your camera, and structuring your answers concisely within the allotted time limits.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Write a function to reverse a string in-place.
Write a function to reverse a string in-place.
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Explain how the SPI (Serial Peripheral Interface) communication protocol works, and describe its advantages ov
Explain how the SPI (Serial Peripheral Interface) communication protocol works, and describe its advantages over other protocols.
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Solve a series of bitwise operations and explain how you would isolate specific bits in a register.
Solve a series of bitwise operations and explain how you would isolate specific bits in a register.
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Explain the difference between stack and heap memory allocation, and how pointer errors can lead to memory lea
Explain the difference between stack and heap memory allocation, and how pointer errors can lead to memory leaks in embedded systems.
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Given a pointer to a struct, how do you safely modify its members without causing undefined behavior?
Given a pointer to a struct, how do you safely modify its members without causing undefined behavior?
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Replace offset paging on the resource feed with keyset
resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.
Approach
- Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
- Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
- Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
- Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.
Follow-up
- The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
- Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?
Find version gaps and relay lag with window functions
outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.
Approach
- Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
- Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
- Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
- Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.
Follow-up
- Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
- Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?
Design a database schema to track patient diagnostic results, device status, and operator logs.
Design a database schema to track patient diagnostic results, device status, and operator logs.
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
What makes software design "good" versus "bad" in terms of scalability, modularity, and testability?
What makes software design "good" versus "bad" in terms of scalability, modularity, and testability?
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
How do you approach designing software that must be highly modular and easily updated without breaking core de
How do you approach designing software that must be highly modular and easily updated without breaking core device functionality?
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
Describe your software development philosophy when designing for high-availability systems.
Describe your software development philosophy when designing for high-availability systems.
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
How do you manage and document system architecture diagrams for cross-functional engineering teams?
How do you manage and document system architecture diagrams for cross-functional engineering teams?
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
p99 jumped on one listing filter while p50 stayed flat
After a release that added an owner_user_id filter to the resource listing, p99 rose from 90 ms to 1.9 s while p50 stayed at 40 ms. Traffic and row counts are unchanged. resource carries the index (tenant_id, status, updated_at DESC, resource_id DESC). The new query filters tenant_id and owner_user_id, orders by updated_at DESC, resource_id DESC, and takes 20 rows. On PostgreSQL, explain the shape of the regression, prove it from a query plan, and give the index you would add.
Approach
- Start from the shape. A flat p50 with a moved p99 means a subset of requests changed cost, not all of them, so the first job is naming the subset. Bucket the endpoint's latency by the tenant's row count; the natural hypothesis is that large tenants are a small share of requests and all of the tail.
- Get the plan for the new query on a large tenant with EXPLAIN (ANALYZE, BUFFERS). Expect an index scan over the tenant's range, a filter discarding most of it, then a Sort feeding the Limit, possibly reporting Sort Method: external merge Disk. Read actual rows on the scan node, not estimated.
- Explain why the existing index cannot serve it. A composite B-tree is seekable only as a left prefix, and with no equality predicate on status the scan cannot treat updated_at as an ordering, because rows in the tenant's range are ordered by status first. Everything matching must be read and sorted before LIMIT 20 can apply, so a tenant with 400,000 rows pays 400,000 rows to return 20.
- Add (tenant_id, owner_user_id, updated_at DESC, resource_id DESC). Equality on the first two columns leaves the index ordered by updated_at within that pair, so the plan becomes an index scan that stops after 20 rows with no Sort node. PostgreSQL can scan a B-tree backwards, so the DESC markers matter only if the two sort columns ever disagree in direction; keeping them explicit documents the order the keyset cursor depends on.
Follow-up
- The endpoint paginates with OFFSET. What does page 500 cost with your index, and what does the keyset version cost?
- How would you have caught this before release, given that a 10,000-row seed database produces the same plan shape at an unnoticeable cost?
Built from the rounds and topics Lumiradx candidates report.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Map the Lumiradx loop
- Write out the reported sequence: Recruiter Screen, Written Technical Assessment, Technical Interviews, Behavioral Evaluations.
- For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.
Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.
02Work Software Design Principles
- Spend the session on Software Design Principles, which Lumiradx candidates report being tested on.
- Write one worked example in Software Design Principles and time yourself on it.
Deliverable: One timed worked example in Software Design Principles.
03Work Scalability
- Spend the session on Scalability, which Lumiradx candidates report being tested on.
- Write one worked example in Scalability and time yourself on it.
Deliverable: One timed worked example in Scalability.
04Work Optics / Optics Design
- Spend the session on Optics / Optics Design, which Lumiradx candidates report being tested on.
- Write one worked example in Optics / Optics Design and time yourself on it.
Deliverable: One timed worked example in Optics / Optics Design.
05Answer out loud: Low-Level & Embedded Programming
- Answer aloud, timed: Write a function to reverse a string in-place.
- Answer aloud, timed: Explain how the SPI (Serial Peripheral Interface) communication protocol works, and describe its advantages over other protocols.
Deliverable: Spoken answers to 2 reported Low-Level & Embedded Programming question(s), under time.
06Answer out loud: System Design & Architecture
- Answer aloud, timed: Design a database schema to track patient diagnostic results, device status, and operator logs.
- Answer aloud, timed: What makes software design "good" versus "bad" in terms of scalability, modularity, and testability?
Deliverable: Spoken answers to 2 reported System Design & Architecture question(s), under time.
07Answer out loud: Behavioral & Process Engineering
- Answer aloud, timed: Describe a complex technical bug you solved in a past project. What was your methodology for diagnosing and resolving it?
- Answer aloud, timed: What would your current colleagues say are your greatest strengths, and what would your current employer miss most about you?
Deliverable: Spoken answers to 2 reported Behavioral & Process Engineering question(s), under time.
Expand any day for tasks and deliverables. Your progress is saved on this device.
Behavioural rounds judge the decision you made and what it cost.
Describe a complex technical bug you solved in a past project. What was your methodology for diagnosing and re
Describe a complex technical bug you solved in a past project. What was your methodology for diagnosing and resolving it?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
What would your current colleagues say are your greatest strengths, and what would your current employer miss
What would your current colleagues say are your greatest strengths, and what would your current employer miss most about you?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
How do you ensure compliance with regulatory requirements and quality standards during the software developmen
How do you ensure compliance with regulatory requirements and quality standards during the software development lifecycle?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Tell me about your favorite engineering project. What made it successful, and what did you learn from the chal
Tell me about your favorite engineering project. What made it successful, and what did you learn from the challenges you faced?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Why do you want to work at Lumiradx, and how does your background align with our diagnostic product lines?
Why do you want to work at Lumiradx, and how does your background align with our diagnostic product lines?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
- 01
Describe a complex technical bug you solved in a past project. What was your methodology for diagnosing and resolving it?
- 02
What would your current colleagues say are your greatest strengths, and what would your current employer miss most about you?
- 03
How do you ensure compliance with regulatory requirements and quality standards during the software development lifecycle?
- 04
Tell me about your favorite engineering project. What made it successful, and what did you learn from the challenges you faced?
How technical is the interview process for software roles at Lumiradx?
The process is highly technical and practical. Depending on the team, you should expect hands-on coding tests (sometimes written on paper or completed online), low-level programming questions, and detailed discussions about software architecture, database design, and hardware integration.
Lumiradx Software Engineer candidate reports ↗Is prior medical device experience required to join the team?
While prior experience in medical devices or regulated environments (such as ISO 13485 or IEC 62304) is highly valued and will set you apart, it is not always a strict requirement. Strong software engineering fundamentals, a commitment to quality, and a willingness to learn the regulatory landscape are equally important.
Lumiradx Software Engineer candidate reports ↗What programming languages are most commonly used at Lumiradx?
The primary languages used are C and C++ for embedded firmware and device-level software, and C# or modern web stacks for desktop applications, connectivity tools, and cloud platforms. You should clarify with your recruiter which stack your target team utilizes.
Lumiradx Software Engineer candidate reports ↗What is the company culture like for engineers?
The culture is collaborative, professional, and mission-driven. Engineers work closely with scientists, hardware designers, and product managers to solve real-world healthcare problems. The environment is structured, reflecting the safety-critical nature of medical diagnostics.
Lumiradx Software Engineer candidate reports ↗What topics does Lumiradx test in interviews?
Lumiradx interviews most often cover Software Design Principles, Optimization strategies, Scalability, Optics / Optics Design, and Research problem solving. The exact emphasis depends on the specific role you apply for.
Lumiradx Software Engineer candidate reports ↗Sources & methodology 3 sources ↗
No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.
- 01Lumiradx Software Engineer candidate reports ↗
Company-reported rounds, questions and FAQ.
Candidate reports · Accessed 2026-09-22 - 02PracHub Software Engineer practice ↗
PracHub practice material, not company-reported.
PracHub page · Accessed 2026-09-22 - 03PracHub preparation framework ↗
PracHub preparation guidance.
PracHub page · Accessed 2026-09-22
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