Daedalus' Blueprint: Why Your Product Requirements Document Prevents You From Building a Labyrinth

Before Daedalus constructed the legendary labyrinth of Crete (see more on the maze and The Product Manager's Role During R&D Feasibility), the master architect understood something fundamental: without a clear blueprint, even the creator becomes lost in their own creation. The most brilliant design becomes a trap when the builder can't remember which corridor leads where, which passage connects to what, or why certain turns were necessary in the first place.

Product Managers (PMs) face the same risk. Start development without a clear Product Requirements Document (PRD), and six months later you're wandering a maze of features, wondering which stakeholder asked for what, why certain capabilities exist, and how you'll ever explain to leadership what you actually built.

TPP vs PRD: What's the Difference?

If you've heard colleagues in therapeutics or diagnostics mention Target Product Profiles (TPPs), you might wonder if you're missing something. You're not.

A TPP is a strategic document used primarily in drug and diagnostic development that defines the ideal commercial and clinical characteristics of a product before significant R&D investment. It answers questions like: What patient population? What performance specifications? What competitive advantage? What clinical claims?

A PRD is the tactical specification document that defines how a product will be built. It translates user needs into functional requirements that engineering can execute against. For RUO tools, the PRD is your primary requirements document.

The key distinction: TPPs describe the destination. PRDs describe the vehicle that gets you there.

Since TPPs are uncommon in RUO product development, this post focuses on building PRDs that keep teams aligned and product buildable.

When Do You Start the PRD?

Your PRD lives at Gate 2, after concept approval but before full development investment. You need enough feasibility work to know what's possible, but you write the PRD before development begins to align what you'll actually build.

Starting too early means specifying features before you understand technical constraints. Starting too late means engineering makes assumptions without your input. Gate 2 is where voice of customer insights (which you've already translated into user requirements) become detailed specifications that teams can execute against.

What Makes a Solid PRD?

A solid PRD answers three fundamental questions for every stakeholder:

For R&D: What am I building, and what defines success?
For Sales: What am I selling, and what can I promise customers?
For Leadership: What am I funding, and how do we measure ROI?

The document itself should contain:

User Requirements (PM-owned): The problems you're solving and success criteria from the customer's perspective. These are technology-agnostic and directly traceable to VOC insights. Example: "Researchers must be able to process 96 samples in under 4 hours with less than 10 minutes hands-on time."

This is where many PMs stumble. They write what they think users need rather than what users actually prioritized. One emerging practice is converting personas into queryable AI assistants (personaAI) that help validate whether requirements actually address target user pain points. Instead of assuming you understand a principal investigator's (PI's) challenges, query your personaAI Assistant: "Describe the operational challenges a PI running a core facility faces when staff turnover is high and instrument training is complex." The AI responds based on training it with real LinkedIn profiles, job descriptions, and communication patterns of actual users in that role.

The output helps you reality-check your requirements. If your PRD emphasizes throughput but the personaAI Assistant reveals core facility PIs lose more time to repeated training cycles than to runtime bottlenecks, your requirements solve the wrong problem. This doesn't replace user interviews. It prevents you from anchoring requirements to your own assumptions before validating them against how your target users actually experience their daily work.

Functional Requirements (R&D-owned): The technical specifications that deliver those user requirements. These are specific, measurable, testable and should tie to user requirements. Example: "System shall automate sample loading via 96-well plate format with barcode tracking. Workflow runtime shall not exceed 3.5 hours for full plate processing."

Technical Constraints: What you're not building or what limitations exist (budget, timeline, technology readiness, patent landscape).

Success Metrics: How you'll measure whether the product delivers on its promise (performance specs, usability benchmarks, customer acceptance criteria).

The co-ownership model matters. PMs own user requirements because you're accountable to customer needs. R&D owns functional requirements because they're accountable to technical feasibility. This division prevents scope creep (R&D can't add features without user justification) and ensures buildability (PMs can't specify impossible requirements).

Who Reviews and Approves?

Your PRD review process should include:

R&D Leadership: Can we build this? Do we have the technology, expertise, and resources? Have we completed patent reviews to ensure freedom to operate?

Sales/Commercial: Can we sell this? Do these capabilities map to real customer pain points and willingness to pay? This consultation is critical because sales will provide revenue forecasts that feed your business case. If they don't believe in the pain points/user needs and product specs, their forecasts will reflect that skepticism.

Project Management: Can we deliver this on the proposed timeline with available resources?

Quality/Regulatory (if applicable): Have we considered validation requirements, safety considerations, or compliance needs?

In RUO development, patent reviews replace the regulatory path clearance common in diagnostics. Before R&D commits to building specific implementations, you need confirmation that your approach doesn't infringe on existing patents or that you have licensing agreements in place.

The approval criteria are straightforward: stakeholders stop finding gaps. When R&D confirms technical feasibility, Sales confirms commercial viability, and Legal confirms freedom to operate, your PRD is ready to guide development.

How Do You Know It's Working?

A solid PRD proves itself during development through fewer change requests. If your team constantly comes back asking "what did we mean by this?" or "can we change this requirement?" your PRD wasn't clear enough or you didn't have adequate stakeholder alignment before approval.

The best PRDs become "flashlight" documents that guide the way for engineering to execute against them without confusion. Sales references them in customer conversations without surprises. Leadership reviews progress without questioning the original scope.

If your Gate 3 review (development complete) reveals a product that looks dramatically different from your Gate 2 PRD, either your requirements process failed or your scope management failed. Both are preventable with a solid PRD and disciplined change control.

The Alternative to the Labyrinth

Daedalus succeeded because he planned before he built. His blueprint kept the labyrinth functional for its intended purpose (containing the minotaur) while remaining navigable for those who needed to use it (guards, prisoners, eventually Theseus with his thread).

Your PRD serves the same function. It transforms abstract customer needs into concrete specifications. It aligns stakeholders before expensive development begins. It creates the shared understanding that prevents your product from becoming a maze even you can't navigate.

The alternative is wandering corridors you built six months ago, unable to remember why certain features exist or who asked for them, explaining to leadership why the product you delivered doesn't match what anyone expected.

Build the blueprint before you build the product.

FAQs

When should I update the PRD after Gate 2 approval?

Only when you discover new information that materially changes scope, such as technical constraints that make a requirement infeasible or customer feedback during alpha testing that reveals a critical miss. All changes require formal change control review with the same stakeholders who approved the original PRD. If you're updating the PRD frequently, your feasibility work wasn't thorough enough before Gate 2.

How detailed should functional requirements be?

Detailed enough that engineering can make implementation decisions without guessing user intent, but flexible enough that engineers can optimize technical solutions. Specify what the system must do and how well it must perform. Leave the how it's built to engineering expertise. Example: Specify "system shall process images in under 2 seconds per frame" not "system shall use GPU acceleration with CUDA architecture."

What if Sales pushes back that the PRD specs won't be competitive enough?

This is a critical Gate 2 conversation that deserves serious attention. Sales often has competitive intelligence and customer feedback that didn't surface during your VOC work. Listen carefully to their concerns.

Start by validating whether there's a genuine market gap. Either your VOC work missed competitive requirements that customers actually care about (go back and validate with target users), or Sales is concerned about specs that customers didn't prioritize in your research. Both scenarios require investigation, not dismissal.

If Sales is right and you've missed critical competitive capabilities, you have three options: extend feasibility work to determine if R&D can deliver those specs within your timeline and budget, adjust your timeline or budget to accommodate must-have competitive features, or document the competitive risk in your business case and proceed with reduced revenue assumptions.

What you can't do is commit to specs that R&D hasn't validated as technically feasible within your constraints. Work with Sales to quantify the revenue impact of including versus excluding the disputed features. If the revenue delta is significant enough to justify extended development or increased budget, make that case to leadership together. Build the business case with the product you can actually deliver on the timeline and budget you have, not the product Sales wishes existed. This is a partnership conversation, not a battle.